WE have received a report by Mr. K. B. Williamson, malaria research officer, Penang (c/o Ross Institute of Tropical Hygiene, London School of Hygiene and Tropical Medicine, W.C.1), on an "Investigation of Ferruginous Waters in relation to the Breeding of Malaria-carrying Mosquitoes". It is a common observation among malariologists in the tropics that waters containing rusty deposits or bearing iridescent surface films of precipitated iron are generally free from mosquito larvæ, and the possibility of utilizing this fact for the control of mosquito breeding has often been mooted. But exact information about the composition of waters of this kind and about the source of the iron has been wanting. Mr. Williamson's report is based on the examination of various types of rusty waters on Hampstead Heath and elsewhere in the neighbourhood of London and around Malvern, as well as upon his experiences in Malaya. It deals mainly with questions of chemistry.