The results of studies on the morphology of juvenile and adult forms of r6 species of moss-mites have suggested some new criteria of division of the family Oppiidae into subfamilies and genera. The criterion of division of the family Oppiidae into subfamilies is the absence (Oppiinae Grandjean) or presence (Oppiellinae nov. subfam.) of setae ta on the notogaster of adult individuals, while the subfamilies are divided into genera depending on the presence or absence of dorsal setae (da, dm and dp). The new division supported by ontogenetic observations makes it possible also for juvenile forms to be classified to the appropriate subfamilies. The division is in line with the differences in walking behaviour: both the juvenile and adult individuals of the subfamily Oppiinae agitate the first pair of legs before putting them down, while members of the other subfamily walk without agitating their legs. Instead of the 51 genera hitherto accepted, the new division of the family Oppiidae proposes only 4, grouped in two subfamilies.