AbstractSexual dimorphism in fourth‐instar larvae and adults was studied forChironomus riparius.Wet weight of fourth‐instar male chironomids was on average 29.4% lower than that of fourth‐instar female chironomids at day 10 post‐hatch of eggs, when organisms were reared individually in 250‐ml beakers with 60 g sediment. This weight differential continued to the adult stage and was enhanced with males weighing 39.8% less than females (dry weight). When animals were reared in groups of 15 larvae per beaker, differences between the sexes in larval wet weight were not statistically significant, but males were still 7.4% smaller than females; however, adult males weighed an average of 42.7% less than females upon emergence. Stage of development (fourth instar) was confirmed by head capsule measurement; no significant differences in head capsule widths were detected between the sexes. Animals reared alone, regardless of sex, weighed more than animals reared as a group, indicating that initial larval densities and size of bioassay container can significantly affect larval growth. The probability of making a type I error in sediment toxicity tests due to sexual dimorphism in weight was estimated to be only 3% when dimorphism was most enhanced, that is, for animals reared individually. The effect of dimorphism on data interpretation when animals are reared in groups of 15 to 50 animals per container is thought to be minimal. It is recommended that both larval weight and head capsule width (millimeters) be measured as end points in sediment toxicity tests to differentiate reduced growth from retardation of instar de