A systematic biosensor is constructed for the estimation of toxic compounds based on photosynthetic activity measurement in Selenastrum capricornutum cells. The photosynthetic response was evaluated as a function of light intensity, cell concentration and initial dissolved oxygen. The inhibitory effect of some toxicants (1-butyl-3-methylimidazolium tetrafluoroborate, methanol) on dissolved oxygen production was also determined. In all cases, a toxic response was detected (i.e. a dose-related inhibition of photosynthetic activity was observed). For the present system, a time of only 2 h was needed to predict EC50 values as compared to 96 h for a conventional algal assay based on algal growth rate. Thus, the developed biosensor was proved to be useful as a rapid and simple test method in environmental toxicity assessment.