It has been pointed out by Graham* that measures designed to bring about the destruction of mosquito larvae may be divided into (a) measures intended to destroy the larvae, and (b) measures intended to destroy their food supply. He points out that very little attention has been paid to the second method, which entails an exact knowledge of the freshwater algae. Graham found that the suspended matter in water in which the larvae of Pyretophorus costalis were breeding could be precipitated by the addition of 3 per cent, of common salt, and that then the larvae became cannibalistic and destroyed one another. He considered that this phenomenon was due to the larvae being deprived of their natural food, the algae, which had been destroyed by the addition of salt. “In lesser concentration,” he concluded, “salt appears to inhibit the growth of very young larvae, probably by diminishing the supply of food, but the development of fully-grown larvae appears to be hastened in a hypertonic medium, and they pass into and through the pupal stage with unusual rapidity.”