The seasonal pattern of the abundance of the planktonic copepod Eurytemora hirundoides shows the highest numbers are associated with temperatures between 10 and 20 degree C. However, for this temperature range, the abundance is higher in spring than in autumn. The evolution of the intrinsic rate of increase (r sub(m), determined in laboratory), as a function of the evironmental temperature, predicts a peak abundance at the end of May which is consistent with observations. The decrease of r sub(m) occurring from 18 degree C cannot be the only explanation for the decline of the summer population. Other factors are involved: oxygen tension, competition. Two types of action of the temperature, direct in spring, indirect in summer, must be taken into account to evaluate the thermal impact of industries using cooling water from estuaries.