The objective of the study was to assess potential deleterious effects of certain toxicants, singly and in combination, to penaeid shrimp. In nature, these shrimp are exposed to combinations of toxicants from industrial and municipal out-falls, from agricultural runoff or from dredge-and-fill operations. The combined toxicities of methoxychlor and cadmium to penaeid shrimp, Penaeus duorarum, were either independent or additive, and varied with the method(s) of bioassay. Conclusions were based on the results of 10-, 25- and 30-day bioassays conducted with the toxicants added singly or in combination to flowing water of constant salinity and temperature. Cadmium, but not methoxyclor, was accumulated by shrimp and methoxychlor appears to influence the processes of accumulation or loss of cadmium from tissues of shrimp.