Abstract: Large glazed earthenware“kongs”of about 25 litres capacity were used for most of the tests:each receiving 10 litres of pond water and the experimental material.The insecticide was obtained from the factory in the form of very dilute“dusting powder”,which is stated to contain 88% Talc,and 12% of the commercial“666”,the latter being only 0.65% pure gamma isomer of hexa-chlorocyclohexane. For preliminary tests we decided to take as a basis for our dilutions the commercial product,which we can term“666 Com.”(=0.065% gamma isomer).We made up a 2% stock solution of“666 Com.”in absolute alcohol. On adding the alcoholic stock solution to the water,drop by drop,an extremely fine precipitate is formed like a faint cloud,which soon becomes dispersed in the water. Concentrated samples of planktonic entomostraca were collected with a coares tow-net,and this was added in approximately equal quantities to the experimental“kongs”:subsequent samples being taken for counting from the 10-litre volume by stirring in alternate directions,and allowing a 25 cc.stop-pered museum jar to fill under water.This sample was formalized,allowed to settle,and about 18 cc.of the supernatant fluid removed by suction,the rest was transferred to 10 cc.centrifuge tubes,plus washings to make up the volume.The whole of the deposit was transferred to a Sedgwick-Rafter Counting Cell,and the number of entomostraca counted under a low power: they were separated into 10 categories,viz.“dead”and“alive”of Daphnia types,Chydorus and Bosmina types,adult Copepods,Copepodites,and Nauplii. The next trials were made with freshly colleted plankton as before, but to obviate sampling errors,the whole of the experimental lot was counted. A series of 25 cc.museum jars were filled with equal amounts of plankton, plus the test solutions,and the whole of this was eventually centrifuged and counted as described above. Inspection of Table Ⅰ would suggest that the lethal dose of“666 Com.” for planktonic crustacea lies near 0.5 p.p.m.,and that 10 p.p.m,is rapidly fatal to them.These figures correspond to 0.0033 and 0.065 p.p.m,respectively of the chemically pure gamma isomer. It is probable that Cladocera are slightly more sensitive to this drug than Copepoda,but all observations showed that the adult free-living copepods and the larger Daphnia-types of Cladorcera died more quickly than the smaller forms. During the series of tests two larval Argulus were found in the plankton hauls,and one of them was placed in 0.2 p.p.m,of“666 Com.”but it showed no reaction to the drug after 3 hours,when it was still swimming very actively, so were the Rotifera and ciliates along with it.It was then transferred to 1.0 p.p.m,and left,without observation,for 2 hours,at the end of this time there was a loss of motility.It is therefore concluded that 1.0 p.p.m,is definitely lethal to Argulus(=0.0065 gamma isomer),and that this is a practical amount to add to fish ponds for the control of epidemics of the fish louse. Three series of tests in the 10-litre volumes were made at first on a mixed collection of shrimps and small fishes:in all these(except the Control)the shrimps died in a few hours.Both the large and small common species of shrimps are about 5 times more sensitive than planktonic crustacea to this drug.In a concentration used for killing Argulus in a fish pond,the entire shrimp population will also be killed within 3 hrs. In the preliminary experiments in 0.4 p.p.m.,some 20 shrimps were removed from the fish“kongs”showing varying degrees of opacity,spreading upwards from the tail.If the opacity has not completely spread over the proximal abdominal segment(s),a few hours in fresh water will show that the phenomenon is reversible.The opacity fades,and with the return of transparency of the muscles,the colouration of the cuticle becomes normal again,and at the same time motility is regained.In those shrimps in which the opacity had reached the thorax,and the legs were consequently pink and immotile,there was no recovery. Apart from the restlessness of the larger,diseased Ctenopharyngodon in high concentrations,which may have been due to a secondary hypersensitivity of the skin due to lesions,we can say that small fishes,at least,are tolerant to“666 Com.”concentrations up to 10 p.p.m.,and that this represents a “safety coefficient”of 10 when we use 1.0 p.p.m.“666 Com.”for killing Argulus on such fishes. In marked contrast to Argulus,the parasitic Copepoda,Lernaea and Ergasilus attached to the fish,and also the free-living males and larvae of certain observed Ergasilidae,were unaffected by concentrations up to 10 p.p.m.“666 Com.”in the case of those attached to the fish,and at least up to 0.5 p.p.m,for the free-swimming parasitic forms.The free-living Copepoda and Cladocera,on the other hand,though less sensitive than Argulus,are certainly all killed by 10 p.p.m,which left the attached female parasitic copepods unaffected. Incidental observations while examining Argulus showed that Rotifera and ciliates were unaffected by“666 Com.”up to 1.0 p.p.m.,.Monogenetic trematodes proved to be highly resistant.