When assessing pesticides toxicity on test organisms, the use of organic solvents is frequently required to formulate solutions of pesticides having low or moderate water solubility. These solvents may influence the results of toxicity tests. This study examined the effects of ethanol and dimethyl sulfoxide (DMSO) on two species of algae, Chlorella vulgaris and Selenastrum capricornuturn. The interactives effects of these organic solvents with various concentrations of atrazine were tested to analyse how these solvents can affect the toxicity of atrazine. Toxicity was measured as the change in chlorophyll (a) content (estimated via fluorescence) in cultures of the test organism that were incubated over a 96-h period at standard conditions of temperature and light. In the absence of atrazine, ethanol was toxic to both algae, yielding significant inhibition of chlorophyll (a) content at concentrations as low as 0.05%. Morever, S. capricornuturn was less sensitive to ethanol than was C. vulgaris. DMSO did not show any toxic effects on either chlorophycea. At concentrations up to 0.5%, DMSO interacted additively with atrazine for both chlorophycea. An additive response was also observed with ethanol towards S. capricornu- turn. For C. vulgaris, ethanol interacted antagonistically at most of atrazine concentrations and gave few additive and synergistic interactions. An additive response of an atrazine-solvent mixture indicate the inherent toxicity of the atrazine. Since DMSO generally gave additive responses for both algae, it can be considered as an adequate organic solvent to use in bioassays.