[en] Artificially fertilized eggs of common flounder, Paralichthys olivaceus and a species of puffer, Fugu niphobles were reared until hatching in tritiated sea water of various concentrations. No significant difference in their hatchability was observed between control and tritiated water groups of 10^-8 to 10^-2 Ci/l. However, a decrease in puffer egg hatchability was observed in tritiated water of very high concentrations, i.e., 1 and 10 Ci/l. Although the eye diameter of the 10 Ci/l larvae was found to be only about 57 per cent that of the control, the body weights of both groups were nearly equal, possibly due to the larger amount of residual yolk in the 10 Ci/l larvae. Naturally spawned eggs of the Japanese Killifish, Oryzias latipes were reared in tritiated water of 10^-2 and 1 Ci/l for eight days and transferred into non-radioactive water. Hatched larvae were fed with powdered artificial fish food and Daphnia for the next 30 days. Comparing with the control group, both survival rates and the mean fork lengths were slightly reduced in the groups treated with the tritiated water, but the differences were not statistically significant. (auth.)