It has been demonstrated that zinc ions inhibits population growth ofEuglena gracilis. Experiments were conducted under laboratory conditions to assay the effects of zinc in varying concentrations on the time dependent growth profile ofEuglena gracilis. The technique consists of growing the organisms in media containing zinc for predetermined times, followed by microscopic examination of cell populations. Results may be rapidly obtained within a few hours in the acute and within 1 or at the most 2 weeks in the chronic tests.