While there is abundant evidence for the effects that seminal fluid peptides and proteins can have on females, we demonstrate for the first time that such peptides and proteins are also transferred by hermaphroditic animals. The reported peptide, Ovipostatin, is produced in the prostate gland and transferred during mating, in the ejaculate, along with the sperm. The bioactive factor from the prostate gland was localized to a prominent HPLC peak, which was repurified to homogeneity. N-terminal Cyclobenzaprine hydrochloride microsequence analyses of the purified fraction demonstrated that it contained a single N-terminal partial 31-residue sequence. The remaining sequence was obtained by 39and 59-RACE, which resulted in the complete cDNA sequence of Ovipostatin. This protein turns out to be a novel substance, since its predicted sequence showed no resemblance with any protein identified to date. that mediates the observed reduction in egg laying. Besides the inhibition of egg laying, we find that Ovipostatin does not affect hatching success of the eggs or lettuce consumption by the recipient. Since the reduction in egg mass production in itself seems maladaptive, we expect that Ovipostatin has another fitness enhancing function. There are several possible scenarios that could provide an explanation here. First, the observed effect may represent a general inhibition of the female function that also inhibits the willingness to further mate. This could function to prevent additional matings with other partners and/or to allow time for the Nisoldipine donated sperm to reach the storage site. Although this would provide a direct benefit for the sperm donor, remating in the female role seems not to be inhibited given that sperm recipients are often inseminated several times in a row by different partners. Second, inhibited egg laying could be an indirect effect. For instance, the presence of Ovipostatin could result in higher paternity by increasing storage of the donated sperm and/or displacing already-stored rival sperm. Alternatively, by postponing egg laying in the partner the latter may be committed to invest more resources per egg, thus increasing egg quality.