The simultaneous effect of sex, initial size, and temperature on growth rates, adult size, and size at large wing-pad (LWP) stage ofHexagenia bilineata(Say) mayflies was investigated. We found that female nymphs grew faster than males; that growth at the constant temperature of 15°C was significantly less than at 22.5 and 30.0°C; and that smaller nymphs tended to grow faster than larger nymphs over a period of 49 days. Size of nymphs at the LWP stage was significantly related to sex, with females being considerably larger. Within the same initial size groups, all nymphs reared at 30°C were significantly smaller at the LWP stage than nymphs reared at 22.5°C and 15°C. Metamorphosis was not successful at 15°C. Adults reared at 30°C were significantly smaller than those reared at 22.5°C. The nearly synchronous emergence times of nymphs initially in the 13.5-mm and 17-mm size classes at both 22.5 and 30°C, suggested that adult tissue maturation was initiated almost simultaneously in nymphs of those two sizes. The lag in emergence of nymphs at the initial size of 9 mm suggested that a size-dependent threshold for adult tissue maturation exists between 9 and 13.5 mm body length. Finally, while 15°C was too low to permit successful emergence, it did not inhibit adult tissue maturation.