Tolerance to cypermethrin is negatively correlated with weight in third instar larvae of the susceptible SUR strain of H. virescens. The relationship is not linear, but decreases sharply above a transitional weight of 19mg. Comparisons of the results of third instar topical bioassays between larvae greater than and smaller than the transitional weight leads to 10-fold differences in the estimation of cypermethrin potency. The reasons for the transitional weight are discussed.