Females of most insect species maximize their fitness by mating more than once. Yet, some taxa are monandrous and there are two distinct scenarios for the maintenance of monandry. While males should always benefit from inducing permanent non-receptivity to further mating in their mate, this is not necessarily true for females. Since females benefit from remating in many species, cases of monandry may reflect successful male manipulation of female remating (i.e. sexual conflict). Alternatively, monandry may favor both mates, if females maximize their fitness by mating only once in their life. These two hypotheses for the maintenance of monandry make contrasting predictions with regards to the effects of remating on female fitness. Here, we present an experimental test of the above hypotheses, using the monandrous housefly (Musca domestica) as a model system. Our results showed that accessory seminal fluid substances that males transfer to females during copulation have a dual effect: they trigger female non-receptivity but also seem to have a nutritional effect that could potentially enhance female fitness. These results suggest that monandry is maintained in house flies despite potential benefits that females would gain by mating multiply.