Oil has been used as a mosquito larvicide for about forty years, since L. O. Howard in 1892 first recommended its use, and a large literature has accumulated on the subject, but the physical and chemical properties which make an oil a suitable material for this work are very imperfectly understood. It is clear that the problem is one that can very favourably be studied in the laboratory, for it requires above all an answer to the question “How do larvae die under oil treatment?” which can only be answered by microscopic examination of the larvae.