Different microalgal species have been used in growth-inhibition tests to determine the toxic concentrations of anionic and non-ionic surfactants to phytoplankton. The species used were selected from different taxonomic groups, all of considerable ecological relevance to marine environments. The toxicity of the C13 LAS homologue to the microalgal species selected was usually one order of magnitude greater than that of the C11 homologue. The toxicity of a commercial LAS mixture to different microalgal species was also checked. For this material and C. gracilis, cellular counting by means of a Neubauer chamber and by use of a flow cytometer were compared; differences between the two methods were insignificant. The toxicity of decaethoxylated nonylphenol non-ionic surfactant to C. gracilis was also checked; the EC50 value for this compound was 1.0 mg L(-1).