Log-phase cultures of Acanthamoeba castellanii, Neff strain, have been maintained at elevated hydrostatic pressures over periods of several days and the population has been recounted at the end of the experimental period. A pressure of 2,000 psi (136 atm) depressed growth of the population, but was quickly reversed on release. A pressure of 4,000 psi (272 atm) severly depressed population growth, and any increase was slight and short-lasting at 5,000 psi (340 atm). Growth of the population was resumed only after an interval of 1 or more days after release.