The potential effects of changes in seawater salinity and temperature on the density and sinking velocity of the eggs of the coastal/estuarine calanoid copepod Acartia tonsa Dana were evaluated in the laboratory using the Percoll density gradient centrifugation method. The developmental stage of the eggs also was investigated to determine if the density and sinking velocity of the eggs changed as the eggs developed. Results of developmental stage experiments for A. tonsa eggs spawned in seawater at a salinity of 15%. and a temperature of 20 °C indicated no statistically significant differences in measured mean egg densities or calculated sinking velocities between the different developmental stage groups (range of egg densities of 1.083 to 1.088 g/ml). Results of salinity experiments indicated that, for eggs spawned at a salinity of 15%. and a temperature of 20 °C, the measured mean egg density was 1.066 g/ml, while for eggs spawned at a salinity of 31%. and 20 °C, mean egg density was 1.086 g/ml. Measured mean egg densities and calculated sinking velocities were significantly different between the two experimental salinity conditions. Salinity crossover experiments, designed to investigate the effects of post-spawning changes in seawater salinity on the density or sinking velocity of the eggs, demonstrated a statistically significant change in density and sinking velocity for eggs which were exposed to different experimental salinities post-spawning. These results indicate osmoregulation by the eggs. Results of the temperature experiments demonstrated that the mean densities and sinking velocities of eggs spawned at a temperature of 20 or 30 °C (salinity of 31%.) were significantly different. For these experiments, eggs spawned at a temperature of 20 °C and 31%. salinity exhibited a mean egg density of 1.087 g/ml. Eggs spawned at a temperature of 30 °C and a salinity of 31%. exhibited a mean egg density of 1.077 g/ml. The results of the salinity and temperature experiments demonstrated that both salinity and temperature significantly affected the density and sinking velocity of the eggs of A. tonsa. At temperatures ≥20 °C, typical sinking velocities