Experiments were conducted to determine the effect of behavior on the accuracy of sampling populations composed of several species of marine copepods and mysids held in a large, enclosed seawater pool. Plankton nets having mouth areas of 1,600, 800, and 400 cm2 were towed in replicate sets during midday and midnight under three light conditions in plankton populations of different densities. Statistical treatment of the results showed that smaller nets were more effectively avoided and that the degree of avoidance varied among species. Estimates of the peripheral escape zone were obtained. More mysids, but not more copepods, were caught in darkness than in light, and avoidance tended to be less in denser populations. Time of day had no effect on avoidance behavior. Escape mechanisms and the possible effect of population density are discussed.