Larvae of the common clam, V. (Mercenaria) mercenaria, were grown to setting, or metamorphosis, at constant temperatures of 30.0[degree], 27.0[degree], 24.0[degree], 21.0[degree]and 18.0[degree]C. The mean setting dates for the various temperatures were found to lie on an almost perfectly straight line. The linear regression line was computed, the resulting equation being Y'' = -1.00 X + 37.91, where Y'' is the predicted setting date and X is the temperature. The regression line is an excellent fit to the experimental data, and interpolation within the 18.0[degree] - 30.0[degree]C limits can be made with a fair degree of confidence. However, extrapolation by several degrees beyond these limits is not justified. Analysis of variance tests showed no significant differences among the 5 temperature groups with respect to the mean length of the larvae at the time of setting. Thus, although the larvae grown at lower temperatures required more time to reach the setting stage, in all cases they attained approximately the same mean length before setting. Therefore, there was virtually no relationship between the temperature and the mean length at the time of setting. Furthermore, there was no apparent relationship between the maximum length at the time of setting and the temperature. The correlation between mean width and mean length at the time of setting was so high (r = .95) that it seemed unlikely that analyses made using width as the variable would give any additional information to those already made with length. There was no apparent relationship found between the shape of the larvae (i.e., the ratio of mean length to mean width) at the time of setting and the temperature. By culturing simultaneously and under identical conditions the eggs and larvae of V. (Mercenaria) mercenaria of Long Island Sound and of southern clams, V. (Mercenaria) campechiensis of the Gulf of Mexico, it was found that the rates of growth of the larvae of the 2 species were practically identical. Furthermore, setting of larvae of both species began at the same time. The results of this experiment contradict, therefore, the widely-accepted opinion that when grown at the same temperature the eggs and larvae of the southern species develop more slowly than those of the northern species of the same genus.