The acute joint toxicity of industrial organic chemicals to the fathead minnow was determined for binary and equitoxic multiple chemical mixtures. Results from binary tests were used to define isobole diagrams. The degree of joint toxic action was determined among 27 chemicals from seven different chemical classes. The slopes of the acute concentration response relationships were quite similar for all test chemicals. This suggests that the mode of acute toxic action for these chemicals is alike though it may not be identical. Intoxication signs of fish exposed to nearly all test chemicals were also similar and indicative of an anesthetic like effect. The results of isobole diagrams for binary mixtures, with 1-octanol as the reference chemical, demonstrated a near concentration additive acute joint action over a wide range of mixture ratios for each chemical from 7 different classes.