Cotton produces a group of four terpenoid phytoalexins at localized sites of infection by Verticillium dahliae. Toxicity of these compounds has been bioassayed by counting conidial germination and determining mycelia viability. This labor-intensive method is not readily adaptable to the study of a large number of V. dahliae isolates. We have adapted a method used to quantitate viable cells. The method utilizes a tetrazolium salt which is modified by various mitochondrial dehydrogenase enzymes into a colored product. Viable mycelia are quantitated on a multiwell ELISA microplate reader. Using this colorimetric method we have determined ED(50) values for dHG and dMHG of 5.2 µg/ml and 24.5 µg/ml, respectively.