The importance of the reaction of mosquito breeding water was established by MacGregor in 1921, and since then observations have been reported from various parts of the world. In the original paper MacGregor showed that, in the laboratory, three species of English mosquitos could survive an alkalinity of pH 9·6 and died in acid water of pH 4·4, while two other species developed in pH 4·4. Later, in Mauritius, he found the local Anophelines in water ranging from pH 9·5 to pH 8·4 and rarely in places giving the reading pH 7·4 ; and maintained that the reaction of any water there was a reliable index of the presence of Anopheline larvae. Senior White, in Ceylon, found the local Anophelines in water with readings from pH 5·8 to pH 8·6 usually, and decided that on the whole the local Culicines had a more restricted range. Buxton, in Palestine, found six species of Anopheles in water varying from pH 9·2 to pH 7·4, and of these only one occurred in water less alkaline than pH 8. He notes that Nablus, where the water supply is unusually acid for Palestine (pH 7 to 7·5), is free from malaria.