Recherches sur la parthenogenese thelytoque de deux especes d acariens oribates: Parthenogenese -The following observations on atavic males in T. leelorum and P. pellifer are reported: 1. They were found in low proportions (1 to 5 %) in all populations studied of both species, although for a given species their frequency varied from one region to another. 2. Like males of bisexual species, they were found to produce spermatophores, albeit to a lesser extent. 3. The atavic males were shown to be formed by parthenogenesis, and they can thus be considered to be somewhat rare examples of an accident of development. From a cytological point of view, they have the same number of chromosomes as females (n = 9). Spermatogenesis is similar to that in bisexual species, but stops at a late stage of spermiogenesis. Their spermatozoa are probably nonfunctional. On histological examination of genital organs, hermaphroditic structures were observed in 2 out of 43 males of T. tectorum.