Field validation of laboratory toxicity data for phytoplankton is uncommon but it is important, since results from standard single‐species laboratory tests are often used in assessing the environmental safety of a chemical. This study reports the short‐term in situ effects of three surfactants on the community structure of enclosed natural assemblages of phytoplankton. Standard diversity and similarity indices, mean species number and density and changes in dissolved oxygen concentration were determined for phytoplankton enclosed in 26‐liter bottles after 10 d of exposure in Acton Lake, Ohio. The first observed effect concentrations based on commonly calculated indices of community structure occurred at 108, 8.4 and 2.9 mg/1, respectively, for the anionic, nonionic and cationic surfactants. In contrast, lower effect levels (<1.0 mg/1) based on changes in density of some major taxa were observed. Since standard algal toxicity tests assume a degree of environmental relevancy, the community effect levels were compared with effect concentrations developed in standard laboratory algal toxicity tests monitoring changes in population growth of single test species. The community effect levels could be predicted in some cases; however, the degree of similarity between the laboratory and field results was very dependent on the surfactant, the laboratory test species and the effect parameter.