Experiments were made with natural populations of both zooplankton and phytoplankton from Southampton Water (England), using an electronic particle counter to measure grazing effects. What appear to have been large differences within and between experiments in size of particles selected are shown to have been mainly a result of variations in total amout of grazing. When this was taken into account it was found that there was no difference in size of particles selected most strongly whether the zooplankton was dominated by copepods or smaller cirripede nauplii. This can be accounted for by similarities in the setous feeding appendages of these two forms. It is suggested that similarity in particle selection would lead to competition between the two grazers if they were not separated seasonally.