Protein-bound sulphydryl and amino groups, and the enzyme L-leucine amino-peptidase were studied histochemically in Aedes aegypti parastized by infective Brugia malayi filarial larvae 9–10 days old. The distribution of these protein-linked groups and of this enzyme in the mosquito thorax is described in detail. The concentration of sulphydryl and amino groups in the flight muscle fibres is similar to that in mammalian skeletal muscle. The presence of larvae within the fibres causes no change in either the concentration or distribution of these groups, except in the rare instances when small patches of amorphous material are formed adjacent to a larva. No aminopeptidase activity is found associated with normal or with parasitized muscle fibres, and the larvae are themselves either completely or virtually negative. The lack of observable histochemical changes correlates well with the absence of histological damage. These results are compared with those obtained in a histochemical study of the effects of the larvae of the nematode, Trichinella spiralis, on mammalian muscle. Possible reasons for the observed differences are discussed.