AbstractLaboratory bioassay experiments were conducted with juvenile rainbow trout (Oncorhynchus mykiss) to determine the acute lethal (96‐h LC50) values of the forest‐use herbicides Garlon 3A®, Garlon 4®, and Vision® with 15% and 10% surfactant formulations, and the threshold concentrations of these herbicides that would cause behavioral effects. A Y‐maze apparatus was used to determine the threshold concentration of each herbicide that elicited either an avoidance or a preference reaction from rainbow trout fry after a one‐hour exposure period. Rainbow trout fry were observed for qualitative behavioral changes (e.g., increased coughing and ventilatory rates, loss of equilibrium, etc.) over a four‐day period of exposure to a wide range of chemical strengths. Nominal concentrations of the herbicides eliciting threshold avoidance reactions in test fish were 150 ppm Vision‐10% surfactant, 54 ppm Vision‐15% surfactant, 19.2 ppm Garlon 4, and 800 ppm Garlon 3A, which were, respectively, 2, 2, 8, and 2 times the 96‐h LC50 values determined for each herbicide. Preferential reactions to the herbicide solutions were not observed. Qualitative behavioral changes were observed in rainbow trout fry at levels from 25% (Garlon 4) to 50% (Garlon 3A, Vision‐10% and ‐15% surfactants) of the 96‐h LC50 values. Nominal concentrations of the herbicides causing threshold changes in fish behavior were 37.5 ppm Vision‐10% surfactant, 13.5 ppm Vision‐15% surfactant, 0.60 ppm Gar