Acrylate and methacrylate esters are reactive monomers that are used primarily in the synthesis of acrylic plastics and polymers. Ninety-six hour flow-through acute toxicity tests were conducted with fathead minnows (Pimephales promelas) using 6 acrylates and 6 methacrylates. Ninety-six hour LC50 values were calculated and then compared to a previously reported structure activity relationship (SAR) for ester narcosis. In addition, behavioral and morphological changes were recorded using a previously developed checklist and were then evaluated using discriminant function analysis. The results demonstrate that acrylates are much more toxic than methacrylates, and 4 to 56 times more toxic than would be predicted by the baseline ester SAR. In general, measured methacrylate toxicity was within a factor of 2 of values predicted from the ester model. Fish exposed to methacrylates were generally hyperactive and underreactive to stimuli. Fish exposed to acrylates were hyperactive and overreactive to stimuli with some eliciting tetany, scoliosis and/or lordosis. The results suggest that methacrylates are acting as narcotics, while acrylates generally act as reactive toxicants. Although methacrylate toxicity can be reliably predicted, the lethality of acrylate esters, which seemingly elicit their toxicity through a reactive mechanism, is underestimated with the current ester narcosis model. Based on the findings a more acceptable SAR for acrylates is proposed.