The sizes of the populations of individual bacterial species diminished following their addition to water from lakes with different trophic levels at temperatures of 5, 10, 15, and 30 C. Some species persisted after their initial reduction in cell numbers, but others were undetectable after 3 to 15 days. The decline of these introduced bacteria was not a result of their inoculation at higher densities than are found in nature. Protozoan predation was a significant factor in the fall in bacterial populations sizes because protozoa increased in numbers as the bacterial density fell, the suppression of protozoa led to the elimination or delay of the decline of the bacteria, and the addition of protozoa to lake water in which indigenous protozoa were suppressed produced the same pattern of bacterial elimination as in untreated lake water. (Copyright (c) Springer-Verlag New York Inc. 1990.)