Abstract: Three isozyme including gloucose phosphate isomerase(GPI),malica dehydrogenase(MDH) and phosphoglucomutase(PGM) were selected,and vertical polyacraylamide gel electrophoresis was used to screen 24 and 10 Brachious calyciflorus clones established respectively from the spring and summer collections from Lake Jinghu.Electrophoresis revealed three electromorph patterns for PGI and six ones for MDH in the clones from both spring and summer collections.Patterns of GPI and MDH enzymes revealed variations existed in the two seasonal populations.Six different clonal groups from the spring population and seven different ones from the summer population were identified by combing isozymes variations.Two clonal groups with same genotype existed in the spring and the summer populations.The frequency of the same genotype in the rotifer population differed with season,with that in the spring population being 0.0833,and the summer population 0.2.The variation in population structure of Brachious calyciflorus in Lake Jinghu from spring and summer could be described by overlap model(incomplete genetic discontinuity).The results of reproductive experiments made at 15℃,20℃,25℃ and 30℃ on thirteen clones representing different composite electromorphs in spring and summer collections showed that population growth rate,mictic rate,fertilization rate and resting egg production of rotifers varied from-0.054 to 1.443/d,0 to 0.88,0 to 1.0,0 to 129.7 ind./10mL·3d,respectively.The responses in population growth rate,mictic rate,fertilization rate and resting egg production of rotifers to increasing temperature were different among thirteen clones,and temperature,genotypes and their interactions all affected significantly population growth rate,mictic rate,fertilization rate and resting egg production,which might be one of the reasons that contribute to maintain the high genetic variations in the rotifer population.The results stated above indicated that the bio-chemically genetic characteristics of B.calyciflorus population in Lake Jinghu differed with season,and the ecological trait including rate of asexual reproduction(with the population growth rate as an endpoint),occurrence,course and result of sexual reproduction-formation and fertilization of mictic females,and resting egg production of B.calyciflorus population differentiated between the two seasons.