Abstract: Experiments on the suspension of "666" in water for the control of Argulus and some larvae of Dytiscidae were carried out both in laboratory and in field. The effective concentration worked out is 1 ppm suspension prepared from the commercial "666" powder which cotains 0.5% of gamma isomer. The suspension of such a concentration causes no ill-effect on fries, fingerling and yearlings of the pond-fishes, nor on the planktonic proto- zoans, rotifers and entomostracal crustaceans. Fingerlings of the pondfishes show no indication of distress in a suspenion of even much higher concen- tration, such as 4 ppm of wettable "666" which contains 6.5% of gamma isomer. The freshwater shrimp appears to be more sensitive to "666" than Argulus and Dytiscidae larvae.