Metamorphosis of fiddler crab Ilyoplax pusilla larvae from megalopal to first crab stage was studied in laboratory experiments under various conditions of salinity and substratum. The hatched zoea metamorphose through five stages before reaching megalopal stage. The megalops were placed in a 20 ml Petri dishes (2-3 megalops/dish), with salinities of 10, 20 or 30%%. Each salinity level was tested either with or without sandy mud substratum. Thirty to 35 megalops were used in each of the six treatments. The experiment was carried out at a water temperature of 28+/-0.5 o C and with daily feeding of the diatom Chaetoceros gracilis. Treatment of 20%% salinity and sandy mud substratum revealed the highest metamorphic rate (87%), while in contrast no metamorphosis was observed at 30%% salinity without sandy mud substratum. Duration of the metamorphosis was about 16 days. Numerous malformed juvenile crabs were observed in treatments conducted without sandy mud substratum.