Abstract: A brief historical account of the work for the family Ergasilidae has been given.In addition to the external and internal morphology of Chinese ergasilids,descriptions on the life cycle of the genus Sinergasilus,including copulation,eggs laying and post-embryonic development have also been delivered.Notes and discussions on the behavior and habitats of ergasilids and on the specificity of the host-parasite relationship have also been made. Altogether 17 species belonging to 5 genera have been reported.Among them 1 genus and 3 species are described as new to science.The known species are Ergasilus anchoratus Mark.E. braini Mark.E. hypomesi Yamaguti,E. magnicornis Yin,E. peregrinus Heller,E. scalaris Mark.E. tumidus Mark. Neoergasilus japonicus(Harada),com.nov.,Paraergasilus longidigitus Yin, P. brevidigitus Yin,Pseudergasilus parasiluri Yamaguti,Sinergasilus polycolpus(Mark.),com.nov.,S. undulatus(Mark.),com.nov.,S. major(Mark.), com.nov. The diagnosis of the new genus,a new combination and 3 new species are summerized as follows: The new genus Neoergasilus is distinguished from Ergasilus mainly by the characteristic first swimming leg,which is much prolonged to such an extent that it reaches to the fourth or fifth thoracic segment at the ventral surface.The spine on the second segment of the exopod becomes swollen into a large thumb-stall-like projection,which lies outside of and paralleizes with the third segment,being more or less longer than the later.Between the exo-and endopod of the first swimming leg there is a triangular tooth pro- jecting from the posterior edge of the basis of the leg. The male is slightly smaller than the female.The maxilliped appears to resemble that of Ergasilus.The triangular tooth,which projects from the basis of the first leg,is also present,but less obvious as in the female.(1)Neoergasilus japonicus(Harada,1930)com.nov. The body is cylindrical and about 0.65—0.85 mm.long and 0.26—0.31 mm. wide.The first thoracic segment is not fused with the head.The four free thoracic segments become gradually tapered toward the narrow fifth segment. The first two abdominal segments are nearly equal in size while the third is the longest.The caudal rami are twice as long as wide,bearing 4 setae,the innermost of which appears stouter and much longer than the others.Each of the egg sacs is shorter than the body,with inclosed eggs arranged in 3—4 longitudinal rows. The first antenna has six segments.The second antenna possesses 5 joints and is provided with a relatively slender terminal claw.The mandibles appear as oblong ovate blades,being fringed with fine feathery setae on both sides; each blade bears a narrow serrated pulp on its horizontal base.The first maxilla appears as an irregular elliptic knob,furnished with two setae,the outer one is longer than the inner.The second maxilla has an exceedingly massive eggplant like basal joint,lying transversely well beyond all the other mouth parts;the terminal joint bends directly forwards perpendicular to the transverse axis of the basal segment,with its terminal portion turning ventrally and forming a cardiac shaped flap,which is armed with numerous short spines. The first swimming leg is characteric to the genus.The fourth leg is extremely small and consists of only one segment.The fifth leg is simple and bearing a lateral seta and two longer terminal setae. The body of the male is very slender,about 0.66—0.68 mm.long.The maxillipeds are 4 jointed,the basal segment is short,about 25.6 μ in length; the second segment is about 51.1 μ and the third one is as long as the basal, bearing a short seta on its distal end.The terminal segment is very long(96 μ)and slightly expanded at the base and bluntly at the tip. A triangular tooth is present on the basis of the first leg as in the female. Both rami of the fourth leg are 1-jointed.(2)Neoergasilus longispinosus sp.nov. The first thoracic segment is separated from the head.The three abdominal segments are distinctly wider than long.The caudal rami are provided with four setae. The first pair of swimming legs are of enormous size,exhibiting the generic characters.The anterior four pairs of swimming legs are biramus; each ramus consists of three segments,except the exopods of the fourth,which are 2-jointed.The specific name "long-spined" refers to the strongly developed long spine on the first joint of the exopods of the second,third and fourth legs, which extends out beyond the level of the posterior border of the third joint. The basal segment of the fifth leg is stout and bears one lateral and two terminal setae.The egg sacs are about half the length of tbe body and each contains 4—5 longitudinal rows of eggs. The general form of the male resembles to the male of N.japonicus except that the exopod of the fourth leg is two jointed and the endopod is 3-segmented.(3)Neoergasilus inflatus sp.nov. The first thoracic segment as wide as the head and not fused together.The other three segments become diminished regularly posteriorly,and the fifth segment has not been observed in this species.The genital segment is wider than long.The abdominal segments are shorter than wide.Each caudal ramus bears four setae,two of which are very short,fine and hardly discernible. The second antenna is relatively stout;its fourth joint is the longest provid- ed with a piece of tile-like chitinous plate on the inner side of the distal end. The terminal claw is short and furnished with a small tooth turned back on the inner margin of its proximal end. The first swimming leg is of enormous size and reveals the generic characters.The exopod of the fourth leg is one jointed and the endopod has two. The fifth leg has not been observed.(4)Paraergasilus medius sp.nov. Numerous female were discovered from the nasal cavities of Mylo-pharyngodon piceus,Ctenopharyngodon idella,and Elopichthys bambusa. The body is slender and distinctly longer(0.9—0.95mm.)than the three known species.The head is clearly separated from the first thoracic segment and its postero-lateral angles of the wall draw out posteriorly into a pair of long spines,which extend nearly as far as to the posterior border of the first thoracic segment.Another pair of triangular teeth,though short and trans- parent,are projected from the posterior border of the head inside to the lateral spines. The first thoracic segment is even broader than the head while the other four successive segments are gradually diminishing in size.The genital segment is slightly wider than long.The three abdominal segments are gradually tapering to the posterior end.Caudal rami are slightly longer than wide and each ramus bears four setae. The first antenna is five segmented;its terminal segment bears four long and two very short setae. The basal joint of the second antenna is broad,only about half the length of the second joint;the second joint is long and slightly tapering to the distal end,and the third joint is about one third as long as the second;the terminal claws resemble those of Paraergasilus brevidigitus in shape but distinctly longer(56—70μ).The middle claw is the longest and the inner one the shortest. The anterior 4 pairs of swimming legs are biramus;each ramus is three segmented except the exopod of the fourth leg,which is two segmented.The fifth leg is uniramus,club-shaped,with one terminal and two subterminal setae. Male—unknown.