The life history of the water-bug, Corixa germari (Fieb.) was studied quantitatively in a Derbyshire reservoir and observations on the biology of the species were also made. The eggs of C.germari are firmly attached to stones and are laid chiefly on the undersides or in crevices. The oviposition rate and egg mortality during 1958 were estimated by direct counts of eggs in the field, the abundance of the adults and nymphs was measured in terms of a Standard Net Sweep and the number of eggs laid per female per season was calculated from these data. By means of these methods, it was found that in I958 the breeding adults gave mean catches of 10 per Standard Net Sweep (43% of the adults were females) and that these adults produced 916 eggs. About 530 hatched and, of the nymphs produced, about 80 survived to become adults in the autumn. About 20 of these survived to breed in 1959. Estimates of the density of eggs laid per m(^2) of substratum in 1958 show that the catch per Standard Net Sweep represents about 1/38 of the number of C.germari per m(^2) of substratum. By weighing the various stages in the life history it was shown that the standing crop of C.germari gives a biomass value of2about 20 g. dry weight per m of substratum in the autumn. This is the highest value reached in the course of the season. C.germari has been shown to live at greater depths than most other British species and this may be a means of avoiding the the effects of wave action. Its ability to exploit deep water is coupled with the fact that it visits the surface less often and swims more rapidly than certain shallow water species.