Crude extracts of Musca domestica L. pupae (48 hrs) of the WHO/IN/M domestica/1 strain, have been subjected to electrophoresis on different supports and stainedfor tyrosinase (o-diphenol O₂ oxidoreductase) activity. The zymograms we obtained havebeen described and discussed. In particular it has been observed that when electrophoretic runs were performed on polyacrylamide gels, the zymogram patterns of Musca were very similar to those earlier shown in Drosophila by Mitchell and Weber (1965) and by Peeples and others (1969 a, 1969 b). Patterns showing more isozyme fractions have been obtained when electrophoresis were performed on starch gels. In fact many individuals possessed 7 catalytically active bands $(A_{1},A^{\prime}_{2},A^{\prime\prime}_{2},A^{\prime\prime\prime}_{2},A^{\prime}_{3},A^{\prime\prime}_{3},A^{\prime\prime\prime}_{3})$ anodically migrating and 3 other bands $(A^{\prime}_{c},A^{\prime\prime}_{c},A^{\prime\prime\prime}_{c})$ moving toward the cathode in an electrophoretic support the pH range of which was 8.65. Electrophoretic heterogeneity principally present in A₂ but sporadically affecting the A₃ and A_c zones, has been detected by screening a total of 164 single pupae homogenates. On the basis of our experiments it has not been possible to understand if this heterogeneity was due to the presence of gene-enzyme polymorphism or to the occurrence, in vitro, of heteromultimeric and homomultimeric equilibria among different A subunits. The presence in the screened population of specimens, apparently possessing a normal amount of well pigmented and hardened cuticulae, but showing completely "null" or "silent" zymograms, could suggest that these enzymes are not the only ones interested in melanin formation and in cuticolar hardening and tanning. However this hypothesis does not take into consideration the possibility that the "silent" zymograms might be due to the sampling of specimens having the same chronological but not the same biological age (occurrence of in vivo asynchronies in the processes of cuticular hardening and tanning).