Interactions between solvents and pesticides were studied. Plates were inoculated with the fungi Pythium-ultimum, Sclerotinia-homeocarpa, and Pestalotia species in the presence 2.5, 5, and 10 parts per million of captan (133062) at five levels of solvent. The median effective concentrations (EC50) to reduce fungal growth were obtained by testing each solvent against all three fungi in the absence of captan. P-ultimum was the most sensitive organism examined: It elicited the lowest EC50 values for all solvents, with the exception of dimethylsulfoxide (DMSO) and dimethylformamide (DMF). The decreasing order of toxicity with P-ultimum was ethanol, DMF, methanol, acetone, DMSO, and hexane. Pestalotia species was the least sensitive, exhibiting sensitivity in the following decreasing order: DMF, ethanol, methanol, DMSO, acetone, and hexane. With DMSO and DMF, the most sensitive species was S-homeocarpa, while P-ultimum was the least sensitive species. For S-homeocarpa, the declining order of toxicity was DMF, methanol, ethanol, DMSO, acetone, and hexane. In general, DMF and the two alcohols were the most toxic and hexane the least toxic solvent. With P-ultimum, captan interacted synergistically with all solvents tested except hexane. Synergistic interactions were observed for S-homeocarpa with all solvents but hexane. Acetone, ethanol, and methanol interacted antagonistically with captan, while hexane, DMSO, and DMF were synergistic. For P-ultimum, the lowest inhibition values usually occurred in the presence of hexane and DMF, while the highest values were observed with DMSO. Captan was generally less toxic to S-homeocarpa in the presence of acetone, methanol, and hexane, and most toxic in ethanol or DMSO. Captan was more toxic towards Pestalotia species when mixed with acetone, ethanol, and methanol, and least toxic with DMSO and hexane. The author recommends that solvent toxicity screenings be performed prior to bioassay experiments, and that the least toxic solvents be chosen for evaluation.