DIEL vertical migration (DVM) of zooplankton is generally considered to be a predator-avoidance strategy: zooplankton migrate to greater depths during the day to reduce their chance of being detected by visual predators (fish)(1). Both phenotypic plasticity and interpopulational genetic variability in DVM patterns exist in zooplankton(2,3). We used large indoor mesocosms ('plankton towers'(4)) to study intrapopulational genetic variation for day depth in a Daphnia hyalina x galeata hybrid population. Clones differing in body size also differed in vertical distribution, with the largest clone residing at the greatest depth during the day. A selection experiment in the presence of fish indicates that alternative antipredator strategies, which involve a complex association between habitat-selection traits and life-history strategies, might be an important factor underlying intrapopulational genetic polymorphism in zooplankton, through a balancing of fitness effects in the presence of visual predators.