The larvicide effect of Bacillus thuringiensis ssp. israelensis was studied on early 4th stage larvae of Aedes aegypti after exposition periods of 24, 48 and 72 h. Among the numerous polypeptides from which the toxin is constructed, that responsible for larvicide activity was identified by means of gel chromatographic separation, paper electrophoresis and gel electrophoresis. The fractions obtained in the course of the examinations contained various quantities of proteins with different molecular masses. The larvicide effect of Bacillus thuringiensis ssp. israelensis is probably associated with sub-units having molecular masses of 130 and 65 kDa, and especially with the 130 kDa protein. A polypeptide with a molecular weight of 27 kDa may also play some part in the manifestation of the insecticide effect. The middle intestines of the larvae used as biotest intensively decompose proteins, so the toxic fragments must be resistant to protease.