In the absence of males, females of Hypoaspis aculeijer lay eggs which develop only into males. However, in the presence of males, the eggs develop into both sexes. The species is therefore presumed to be arrhenotokous (haplodiploid). Laboratory experiments indicated that (1) H. aculeijer is not cannibalistic except under extreme conditions of hunger, (Il) there is no evidence of oophagy, (III) the mite does not overkill its prey, (IV) with large Sinella coeca as prey the efficiency of capture increases with predator density whilst with easily caught prey the efficiency decreases, and (V) large Hypogastrura denticulata reduce the predator s longevity and fecundity. The criteria for a species to reach the eusocial threshold (WILSON, 1971) are reviewed. Although severa! generations of mites can overlap, there is neither co-operative broad care nor the existence of a reproductive caste. There is, however, apparently no harassment of the young. lt is suggested that mites in the family Dermanyssidae, especially the species associated with vertebrale nests, are intermediate subsocial.