Carboxylesterase (EC 3.1.1.1) isozymes of the adult and larval housefly Musca domestica, were separated electrophoretically on polyacrylamide gels. Of the 10 isozymes present in the 17,500 g supernatant of the whole fly homogenate, 9 were detected in the gut and the thoracic muscles. Distribution of isozymes was as follows: isozyme E 1 and E 10 in the foregut as well as in the combined mid-and hindgut preparation, and E 2–8 and E 10 in the thoracic muscles. On the basis of the molecular sieving effect of the gels of different pore sizes, the approximate mol. wt of the isozymes E 1–9 were found to be in the range of 67,000 (±3000)–300,000. Isozyme E 10 had the highest molecular weight of ca. 1 × 10 6 . Ten isozymes were resolved from larval tissues, and these corresponded to E 1 and E 3–10 of the adult, and another isozyme E x with no correspondence to any of the adult isozymes. Isozyme E 2 was absent. Distribution pattern of larval isozymes was as follows: E 1, E 3–7 and E 9–10 in the gut; E 1, E 6–8 and E 10 in the muscles; E 1, E 3–8, E 10 and E x in the fat body. Densitometric scans showed that the activity detected histochemically and by pH-titrimetric method ( Ahmad, 1970a) was due primarily to isozyme E 1. Although nothing further is known concerning this isozyme, its wide distribution in larval tissues and restriction to gut in the adult stage may be biologically significant. The molecular diversity and varied tissue pattern indicate several rôles for carboxylesterase in the housefly. The physiological rôles discussed are: (1) regulation of JH titre, (2) mobilization of fat in fat body, (3) energy-related catabolism of fatty acid esters in flight muscles, (4) cuticular wax synthesis and transport, and (5) degradation of undesirable, metabolically inert esters.