Experiments investigating the effect of a periodic influx of new species on the diversity of protozoan communities are reported. In one experiment the data suggest that periodic influx resulted in a considerably higher species diversity than in systems to which no species were added. In the second experiment the periodic influx of species did not markedly affect the number of species present. This may be due to the fact that the first experiment was set up with water from Copper Creek, Colorado, a stream low in protozoan species and nutrients, while the second was set up with water from Douglas Lake, Michigan, a lake comparatively rich in protozoan species and nutrients. The latter may have had all the species the system could accomodate (there were 14 detectable species at the start) while the former (with no detectable species at the start) had unoccupied niches. Two major tentative conclusions may be drawn from these experiments: (1) invasion pressure does not increase the number of species beyond a certain point and (2) even in the absence of competition only about half of the invading species were successful.