32 protozoan-free 51 x 76 x 64 mm polyurethane foam units were individually anchored to a line at a depth of approx. 30 cm from the bottom in 2.5 m of water in Douglas Lake, Cheboygan County, Michigan. At weekly or semi-weekly intervals a pair of substrates was harvested and not reused. The term harvest is used to indicate sampling which removed most of the liquid-containing protozoa from the substrate. These results were compared with studies of surface floats and natural substrates. The most significant results of this study are: (1) pioneer colonizers appear to persist longer than colonizers of areas already inhabited by established communities; (2) the same species may colonize artificial substrates placed in different areas of a lake but may do so at different times; and (3) natural substrates and artificial substrates may have many species in common.