Last year an attempt was made to collect the mosquito larva; breeding in the vicinity of the Institute at Yaba, near Lagos, and to identify them with their imagines. This year it was decided to conduct a similar investigation in the township of Lagos itself. My attention had been directed to the importance of the subject by the occurrence of an outbreak of Yellow Fever in Sierra Leone and the Gold Coast; and further, I had been requested to suggest a set of instructions such as would enable the native Sanitary Inspectors in Lagos to identify the larva of Stegomyia fasciata.