Algal toxicity testing is not new, but only within the past few years have data from such testing been used to help set standards for allowable contamination. Early toxicity testing with marine algae used a few planktonic species with inhibition of growth as the primary endpoint. Results obtained from these algal tests established a reputation for insen-sitivity to toxicants relative to animals. Work with vegetative growth of marine seaweeds from intertidal areas also led to conclusions of insensitivity to toxicants. Based on this reputation, marine algae as a group have been considered nonessential for assessing effects of pollutants on the marine environment. Within the past five years, tests with different species of microalgae and reproductive tests with marine macroalgae have come into wider use. These new species and test method endpoints have shown that plants can be more sensitive to toxicants than some of the most sensitive marine animals. It has been proposed recently that the saltwater “Criterion Continuous Concentration” for water quality criteria documents for thallium and acenaphthene be set with reproductive data from the kelp, Laminaria saccharina; which was the most chronically sensitive species tested for both chemicals. Plant tests also are now being required for some National Pollutant Discharge Elimination System permits for the marine environment.