A study is made of the morphology and postembryonic development of Rhizoglyphus echinopus (Fumouze & Robin), a species of Acaridida (Actinotrichid mites). The study is based on the same methods and principles as the author's previous studies of mites and other groups of Chelicerates. Particular attention is paid to the homology of the setae of idiosoma and appendages, and to the postembryonic development of the paraproctal setae. A new interpretation is given of bursa copulatrix and fertilization.