Cement gland protein in male and inseminated female individuals of an acanthocephalan parasite of fish, Pomphorhynchus laevis (Müller, 1776), was localized by immunohistochemistry using an antibody specific for cement protein. Male P. laevis possess 3 pairs of round to oval cement glands ranging from 0·5 to 0·9 mm in length and from 0·3 to 0·7 mm in width. Each gland has an outer portion containing nuclear fragments and other cellular organelles surrounding a space for storage of gland products. Very little work has been carried out on the nature of the cement gland secretions. We have previously reported that the major component of cement is a protein with molecular weight of 23 kDa; in fresh glands it is white in colour. Immunohistochemical studies herein reported were carried out using a polyclonal antibody raised against purified P. laevis p23 cement protein (anti-p23PL). Localization of p23 cement protein at the light microscope level, by means of the anti-p23PL antibody, shows that p23 is present within the cytoplasmic layer of the gland as well as in the gland duct lumen. Interestingly, the p23 cement protein was also identifiable at the posterior ends of females retaining the cap. Positivity to anti-p23PL antibody was obtained not only in the external part of the copulatory cap, but also within the vaginal tract and at the base of the uterine duct. Thus, we report herein the first photographic evidence that the copulatory cap is not a simple gonopore lid but it is really an intravaginal plug.