The marine diatom, Minutocellus polymorphus, is suited for use as a test species in algal toxicity tests. Its responses to 19 toxicants in a modification of Aquil, a chemically defined phytoplankton culture medium, compared favorably with those of Skeletonema costatum, a common test species. The alga grows rapidly, allowing a test duration of 48 h, thus minimizing complications due to toxicant degradation, volatilization, and adsorption to test vessel walls. These and other characteristics argue strongly for use of M. polymorphus as a standard organism in algal toxicity tests.