The population level and potential rate of increase of a population of houseflies, Musca domestica L., on Grand Turk Island was studied for about 2 years to determine what changes occurred when no control was applied (the first year) and when the population was totally suppressed by the application of chemosterilant baits (the second year). During the first year the population was relatively stable (the highest density occurred in June and the lowest in January) and the difference between the high and low levels was only 3-fold. When chemosterilant baits were applied over a 40-week period and total suppression was achieved, calculations based on the sterility achieved and the resulting control revealed that the population showed potential rates of increase ranging up to 4-fold during the period from June through October and from 4-fold to 11-fold from December through April. Thus, in the field, the potential rates of increase of the housefly are low despite its high biotic potential.