The histochemical distribution of acid phosphatase in microfilariae and in the larval stages of 4 mosquito-borne filariae, Wuchereria bancrofti, Brugia malayi, B. pahangi and Dirofilaria immitis, was studied using the naphthol AS-TR-hexazonium technique and light microscopy. Accurate differentiation between microfilariae of the 4 spp. could be made on the basis of their patterns of acid phosphatase activity. In contrast to microfilariae in the blood, the larval stages in the mosquito exhibited different patterns of acid phosphatase activity which were characteristic for each developmental stage. In the 1st-stage larva maximum acid phosphatase activity was found in the anal vesicle, the growing anal membrane (anal plug), buccal cavity, forming intestine and rectum. In the 2nd-stage larva acid phosphatase activity was present throughout the alimentary canal, particularly in the section of the intestine and rectum. In the infective 3rd-stage larva the whole body stained densely red. The reaction for acid phosphatase in the excretory cell complex of W. bancrofti and of both species of Brugia gradually decreased in intensity and disappeared completely towards the end of the 1st-larval stage, whereas in D. immitis a strong reaction in this area persisted throughout the larval life in the mosquito. The presence or absence of enzymic activity in the excretory cell complex and in the amphids of the developing larvae can be used as an adjunctive diagnostic method.