Individual and combined effects of phosphate (P) and zinc (Zn) on the abundance of dominant algae and protozoa in a community were observed. Nutrient-diffusing artificial substrates were colonized in Douglas Lake, Michigan, and then placed in laboratory microcosms containing one of five Zn treatments (control, 0.1, 1.0, 3.0, and 10.0 Zn mg/l). After 1 week of exposure in the laboratory, the substrates were scraped and algal and ciliated protozoan abundances determined. Ten of thirteen algae and five of eight ciliated protozoa responded to experimental treatments. Some algae (diatoms and green algae) and ciliated protozoa were stimulated by high P; some were stimulated by intermediate P; and some were inhibited by high P. One alga and four protozoa responded positively to Zn. Two algae and three protozoa responded to a significnat interaction between P and Zn so that abundances were from 3 to 19 times higher than the added effects of individual P and Zn treatments. Total algal abundance was increased by high P, and total protozoan abundance was increased by intermediate P but at control levels for high P. The number of protozoan species was increased by P. Total algal abundance was increased by combinations of Zn and P, and the number of protozoan species was decreased by combinations of Zn and P. Altered abundance by combinations of Zn and P had not been demonstrated for a community of algae and protozoa previously. Although concentrations of Zn were initially above the level considered safe by the U.S. Environmental Protection Agency, many factors may prevent Zn stress.