In the present paper we describe some experiments on the mite Macrocheles matrius aimed at investigating: a) the possible role of sperm exhaustion in the laying of unfertilized eggs in the last part of the female reproductive period; b) the relationship between age and female receptivity and inseminability. The results obtained show: 1) the number of fertilized eggs laid by a single female does not increase if she mates more than once during her life; 2) with increasing age the receptivity and the inseminability of the females gradually diminishes (under our laboratory breeding conditions no females were fertilized two weeks after reaching adult stage); 3) the laying of unfertilized eggs during the last part of the reproductive period is the rule and also occurs in females inseminated for the first time when relatively old (e.g. 10-14 days after reaching adult stage). The following hypotheses are advanced: a) though the females of M. matrius may mate several times they are only fertilized by the sperms of a single male, probably the first mate (monogamy); b) with increasing age the females undergo physiological and/or behavioural changes which, besides making them less receptive and inseminable, also result in the parthénogenetic development of the eggs in spite of the possible presence of spermatozoa.