Among larval populations of the mosquito Opifex fuscus Hutton from supralittoral pools on the east coast of South Island, New Zealand, some individuals in the second, third and fourth instars had only simple hairs in their mouth-brushes, others had pectinate bristles. All first-instar larvae had simple hairs, and samples of these from the same pool were reared in the laboratory at 25°C. Of 13 fed on dehydrated blood serum, only two developed pectinate bristles, but of 15 fed on fish food (particle size 0·1–0·6 mm.), all but one did. Of a later sample of 16 reared at room temperature on fish food in the first two instars and then on the blood serum, all developed pectinate bristles in the second instar but had reverted to simple hairs by the fourth. In nature, larvae of O. fuscus observed to be feeding by filtering were predominantly of the simple-hair type and those browsing of the pectinate-bristle type. Environmentally controlled dimorphism may increase the probability that an individual will develop to maturity.