Colonies ofChironomus tepperilarvae derived from wild and laboratory strain egg masses were cultured at 17.5 and 25°C with a 15:9 (L:D) h. Morphometric analysis of sequential larval samples identified four larval instars which can be identified reliably even if populations are reared at different temperatures or derived from different genetic lineages. Fourth instars show a bimodal frequency distribution for cephalolabial length, reflecting prepupal sexual differentiation. Third and fourth instars raised at 17.5°C had significantly larger head capsules (based on cephalolabial length) than those reared at 25°C. Larval growth obeys Dyar's Law, with a regression ofy= −1.339 + O.223x, wherex= instar andy= log10cephalolabial length (in mm). Culturing larvae at 25°C is recommended for the rapid production of fourth instars for use in bioassays. When younger larvae are required, culturing at 17.5°C is more appropriate because early instars last much longer at the lower temperature.