The egg attachment system of an estuarine crab Sesarma haematocheir is formed on the maternal ovigerous hairs just after egg laying, and slips off these hairs just after hatching. The stripping is caused by an active factor that we call OHSS (ovigerous-hair stripping substance), which is released by the embryo upon hatching. OHSS was purified, and its active form had a molecular mass of 25 kDa. The cDNA of OHSS cloned from an embryonic cDNA library was 1759 bp long, encoding 492 amino acids in a single open reading frame (ORF). The C-terminal part of the predicted protein was composed of a trypsin-like serine protease domain, with homology to counterparts in other animals of 33-38%. The predicted protein (54.7 kDa) secreted as a zymogen may be cleaved post-translationally, separating the C-terminal from the N-terminal region. The OHSS gene was expressed in the embryo at least 2 weeks before hatching. Expression was also detected in the zoea larva 1 day after hatching and in the brain of the female. However, it was not detected in the muscle, hepatopancreas or ovigerous seta of the female. Ultrastructural analysis indicated that the material investing maternal ovigerous hair, i.e. the outermost layer (E1) of the egg case, is attached at the special sites (attachment sites) arranged at intervals of 130-160 nm on the hair. It is suggested that OHSS acts specifically at these sites, lysing the bond with the coat, thus disposing of the embryo attachment system. This enables the female to prepare the next clutch of embryos without ecdysis.