The relative abundance of different‐sizedAnisops wakefieldiin a small, fishless pond was determined from sweep‐net samples taken along one open‐water and two shoreline transects at noon on two dates, and at noon and midnight on a third date. During the day, large individuals occurred only along the shore, whereas smaller ones occurred both along the shore and in open water. At night, large individuals also occurred in the open water. The diurnal size‐class segregation suggests that young individuals migrate into less preferred, open water to avoid interference and cannibalism from larger individuals. The shoreline may be preferred during the day because it provides greater food resources or a refuge from visually feeding avian predators. Laboratory experiments in containers with floating aquatic plants at one end complemented the field data. When alone, both small and largeA. wakefieldisought cover during the day and moved into open water at night. During the day, the presence of large individuals induced small ones to move into open water. The results of this study on a notonectid population are consistent with some earlier ones in demonstrating that interactions between large and small size classes may lead to pronounced spatial segregation of size classes, and show for the first time that spatial distributional patterns during the day and night may be very different.