Seasonal densities of Eurytemora affinis, a calanoid copepod in the Chesapeake Bay, seem to be controlled by temperature and salinity. To examine the role of osmotic stress we analyzed protein synthesis under various conditions of temperature and osmotic stress. Adult females were exposed in groups for 5 hours to different temperature and salinity regimes in the presence of isotope-labelled amino acid. Newly synthesized (stress) proteins could be separated and identified using polyacrylamide gel electrophoresis and autoradiography. The protein profiles occurring in copepods experiencing osmotic shock alone were different from those of control animals. Copepods transferred to lower (2 and 5‰) and higher (15 and 20‰) salinities showed differences in the up- and down-regulation of specific proteins. Concurrent heat stress changed these protein patterns. Animals experiencing osmotic and heat shock at the same time exhibited enhanced expression of another set of proteins. Variation in induced proteins occurred among individuals.