Larvae of Rhithropanopeus harrisii (Gould) were reared from hatching to the first or second crab stages in 11 combinations of salinities and cyclic temperatures (5, 20, and 35~ S at 20 ~ to 25oc, 250 to 3OOC, and 30o to 35oc; 25~ S at 20 ~ to 25oc and 30 ~ to 35oc). The larvae survived to the megalops and first crab stages in all salinities and cycles of temperature other than 5~ S at 30 ~ to 35oc. The best survival to the megalops (94%) and first crab (90%) stages occurred in 20~ S, 20 ~ to 25~ In all other combinations of salinities and temperatures there was a reduction in survival to the first crab stage. The duration of the larval stages was affected significantly by temperature, whereas the effect of salinity on the mean days from hatching to the first crab stage was not consistent at the different temperature cycles. Development to the first crab stage required the shortest time in 20~ S, 30o to 35~ (mean 12.3 days), and the longest time in 5~ and 35~ S, 20 ~ to 25oc (mean 22.6 days and 21.6 days, respectively). Megalops larvae reared in 35Z S at all cycles of temperature, as well as larvae in 20Z and 25~ S, 3bo to 35~ showed a high percentage of abnormality, with the highest percentage occurring in 35~ S, 30 ~ to 35oc. It appears that larval developmentof R. harrisii is strongly influenced by environmental factors and not solely related to genetic differences.