A simple marine algal bioassay method is described for short- and long-term studies on pesticides and industrial wastes. It can be used for rapid screening of a variety of substances with single-species and multiple-species tests and gives relative toxicities of the pollutants tested. Algae are grown in optically matched culture tubes that fit directly into a spectrophotometer, allowing population density to be estimated by absorbance without removal of samples, 96 h EC50 values for some pesticides and the diatom Skeletonema costatum are: EPN, 340 micrograms/l; carbophenothion, 109 micrograms/l; DEF, 366 micrograms/l; ethoprop, 8.4 milligrams/l; methyl parathion, 5.3 milligrams/l; and phorate, 1.3 milligrams/l. Presence of the chelator EDTA in medium had no effect on toxicity of carbaryl to S. costatum, Nitzschia angularum, Chlorococcum sp. and Chlorella sp. Liquid industrial wastes either stimulated growth, inhibited growth, or stimulated growth at low concentrations but inhibited it at higher concentrations. In mixed-species studies with the herbicide neburon, presence of a resistant species protected the sensitive species. Liquid industrial waste from a paper products plant caused changes in relative numbers, as compared to controls, when S. costatum and Porphyridium cruentum were grown together.