Two species of mysid shrimp, the sub-tropical Mysidopsis bahia and the northern temperate Mysidopsis bigelowi, were exposed simultaneously to cadmium (as CdC12) in a continuous-flow bioassay system to determine the effect on survival and reproductive success. Temperature and salinity were maintained at 21 plus or minus 1lC and 30% respectively. The 96-h LC50 was 110 microgram per liter for both species. The 23-day life cycle LC50 for M. bahia was 19.5 microgram per liter and for M. bigelowi the 27-day LC-50 was 14.8 microgram per liter. At 10 microgram per liter a series of morphological aberrations were observed in both species at the onset of sexual maturity. Carapace malformations apparently prevented molting after the release of the initial brood and resulted in death of brooding females. As a result, although the initial reproductive rate at this concentration was successful, successive broods could not be produced. For both species in this study the no observed effect concentration was 5.1 microgram per liter; the effect concentration was 10.0 microgram per liter. Mechanisms were postulated in this study to explain the effect of cadmium on the molting process and on calcification and enzymatic reactions of osmosis.