Six sets of ten polyurethane foam substrates were placed at different times and at two different sites in Douglas Lake, Michigan. After various periods of exposure, sets of ten substrates were sampled and the composition of the colonizing protozoan assemblages determined. Cluster analyses indicate generally good replication between substrates within a set. Differences between long exposed sets (3 months - 1 year) and relatively short exposed sets (9 days - 3 weeks) were most pronounced. Fall overturn appears to stimulate an increase in mean species numbers but reduces replicability.