28 protozoan free polyurethane foam substrates were placed in two ponds, about one meter apart, with markedly different pH and hardness. The protozoan species which colonized these substrates were studied 7 times at weekly intervals. Of the 202 species identified from the bog pond (pH 5.8-6.0, hardness 17.1 to 34.2 ppm) 112 occurred in more than one weekly harvest (or sampling) and 90 were found only one week. Of the 176 species from the pit pond (pH 8.0-9.0, hardness 102-154 ppm) 110 were recurring and 66 were non-recurring. 13 species were present continuously in both ponds from weeks 3 through 7 and 8 species occurred 50% or more of the time in one pond and did not appear at all in the other. Possible explanations for these similarities and differences are discussed.