Nine samples of waste oils have been examined and after blending with kerosene and Diesel oil their suitability as larvicides was determined. It is concluded that blends containing 25–30 per cent. of waste oil are not so effective as an approved commercial larvicide at the same dosage but that with a higher dosage, which is still within the range commonly applied in the field, i.e. between 4 fl. oz. and 6·7 fl. oz. per 100 sq. ft. (Ramsay & Carpenter, 1932) they give good results. The higher dosage is only necessary when Culicine larvae have to be controlled. Other blends containing 10–20 per cent. of waste oil are just as effective as an approved commercial sample.