In the oribatid mite fauna of Cat Tien National Park (southern Vietnam), we discovered four species, each representating a different genus of the family Nanhermanniidae, and eight species representing four genera of Lohmanniidae. Our records of Cosmohermannia robusta and Masthermannia mammillaris (Nanhermanniidae) are the first for the respective genera from Vietnam; the lohmanniids Meristacarus sundensis, Mixacarus exilis, Papillacarus ramosus and P. cornutus are also newly reported from this country. The morphology of juvenile instars of Cosmohermannia robusta is described and illustrated. The juve-
nile instars of Cosmohermannia differs from those of Nanhermannia by the morphology of prodorsal and gastronotic setae, the presence two large latero-posterior body apophyses, the epimeral formulae, and the length of some epimeral setae. Diagnoses of juvenile instars of the family Nanhermanniidae and the genus Cosmohermannia are proposed. Papillacarus polygonatus sp. n. is described from dark loamy soil of a lagerstroemia forest; it differs from all other species of the genus by its larger body and three different forms of neotrichous setae.