Ctenophores and other gelatinous predators in the ocean are ephemeral in their spatial and temporal distributions, but often have a dramatic impact on the dynamics of their prey populations. Considerable information is currently available on the functional and numerical responses of several nearshore ctenophore species; however, an understanding of their potential for shaping the structure of natural zooplankton assemblages requires further information on the selective nature of their predation. Prey selection by the tentaculate ctenophore Pleurobrachia bachei was examined in the laboratory with predation experiments and videotaped observations of predator and prey behavior. Prey types offered to the ctenophore included each of the developmental stages of Calanus pacificus, adult Acartia clausii, and adult Pseudocalanus sp. Pleurobrachia exhibited an unusual, bimodal pattern of prey selection on the different developmental stages of Calanus. Calanus' vulnerability increased through the naupliar stages, dropped at the first copepodid stage, and then rose again throughout the subsequent copepodid stages prior to another decline at the adult stage. Adult Acartia and Pseudocalanus were found to be among the most vulnerable of the prey types offered to Pleurobrachia. We conclude that Pleurobrachia's diet is determined by the relative availability and vulnerability of prey, and that vulnerability to the ctenophore can be predicted from prey swimming speeds and the susceptibility of prey after encounter.