1.Hypoaspis aculeifer (Canestrini) is a predatory mesostigmatid mite widely known from Europe, where it occurs in various habitats, but it has only recently been noted from North America. It occurs .in soil and litter in south-west Quebec. It has probably been overlooked elsewhere. 2 . H. aculeijer has been reared in the laboratory on Isotoma notabilis (Collembola) and Tyrophagus putrescentiae (Acarina), the latter of which it prefets. It is known to feed only on living prey. 3· The various stages of the life cycle are described and their rates of development and reproduction at 26, 17 and noC. are given. The larva is figured for the first time and the changes that it undergoes on entering the protonymphal stage are noted. The larva does not appear to feed. 4· The setal arrangement on the legs of the adult is noted and illustrated. 5· The relationship between the life histories of H. aculeifer and the species used as food in the laboratory are briefl.y discussed. 6. H. acttleijer probably does not control naturally occurring populations of Tyrophagus.