1. The larvae of two chironomid species, Tanytarsus brunnipes and Anatopynia nebulosa, living in streams consume more oxygen than the closely related Chironomus longistylus and Anatopynia varia from still water.
2. The oxygen consumption of the two stream species falls as the oxygen content of the water diminishes, whereas that of the two still-water species remains approximately constant until the oxygen content has fallen to a low value.
3. Of the larvae of ten chironomid species, those from streams are much less resistant to anaerobic conditions than those from still water.
4. Stream chironomids have a lower thermal resistance than still-water forms.