Eggs of the monogenean Heterobothrium okamotoi were collected immediately after deposited in Petri dishes, and incubated at 15°C, 20°C and 25°C . The average time required for egg hatching was 11.8 days at 15°C, 7.0 days at 20°C and 5.3 days at 25°C. Effects of different salinities (0, 6.7, 13.4, 20.0, 26.7 and 33.4 ppt) on hatching was examined. Hatching was not affected at the salinities of 6.7-33.4 ppt, but oncomiracidia hatched at 6.7 ppt were apparently inactive, probably because of the low salinity. No embryonic development was observed in the eggs in distilled water, and the embryos eventually died. The morphology of oncomiracidium was described. The oncomiracidium of H. okamotoi and those of previously described Diclidophora species, both in the family Diclidophoridae, had many morphological features in common. The H. okamotoi oncomiracidium could survive much longer than other monogenean larvae, with an average life span of 9.1 days, 7.3 days and 4.7 days at 15°C, 20°C and 25°C, respectively, suggesting that it can retain its infectivity much longer than other monogeneans.