Vertical gradients in substrate-associated protozoan community structure in a stratified freshwater lake were investigated using polyurethane foam substrates. Samples were collected at depths of 0.2 m, 6 m, 10 m, 15 m, and 20 m at 9 days, 16 days, 23 days, 30 days, and 37 days after substrate placement in the lake. Protozoan species lists were made for each sample using standard keys. Physical chemical data were collected concurrently. On the basis of number of species and kinds of species two similar groups were evident: epilimnion (0.2, 6 m, 10 m) and hypolimnion (15 m and 20 m). Although initial rates of species number increase through colonization were similar at all depths, colonization continued much longer in the epilimnion, resulting in a significantly greater number of species on equilibrium substrates at MacArthur-Wilson equilibrium. Per cent overlap of kinds of "resident" species was high within a subgroup and low between groups.