The morphological ontogeny of Melanozetes avachai n. sp. from Kamchatka Peninsula (Russia) is described and illustrated. The juveniles of this species have a humeral organ and a humeral macrosclerite, which occur in most Sphaerozetinae (Ceratozetidae), but in the larva seta c1 inserts on microsclerite, rather than on the humeral macrosclerite, which is unique in Melanozetes. The larva lacks the gastronotal shield and the gastronotal setae are inserted on microsclerites, which is unique in Melanozetes; other species have these setae inserted on the gastronotal shield. The juveniles of M. avachai also differ from congeners by the length and shape of some gastronotal setae. All instars of M. avachai have femora I and II uniquely oval in cross section; in all other species of Melanozetes at least femur II is flattened, with a ventral carina. The adult of M. avachai differs from congeners by its larger body size and longer prodorsal seta le. The translamella is variable in M. avachai; usually it is absent, but may be incomplete, or present. The diagnosis of Melanozetes is modified and enlarged with the morphological characters of juveniles.