Since Ireland is the only country in which Pomphorhynchus laevis (Acanthocephala) uses brown trout (Salmo trutta) as its preferred definitive host, the population biology of the parasite in this host was investigated thus enabling a comparison to be made with data collected on P. laevis from other hosts, particularly the cyprinids, chub and barbel. Over a period of 12 months, 549 brown trout were caught from two lakes, Lough Feeagh and Bunaveela Lake, in the Burrishoole River system, Co. Mayo, Ireland. The parasite component community was dominated by a single species, P. laevis. Fifty eight percent of the trout sample were infected with the acanthocephalan and the mean abundance (+/- SD) was 3.1 +/- 5.1 The relationships between the prevalence and abundance of P. laevis and season and site of host capture and host age and sex were explored. As single factors one of these parameters emerged as significant contributions to changes in parasite abundance although some interaction terms proved to the significant. A random sample of over 700 P. laevis parasites were subjected to further investigation and their size, position in the intestine and maturity status are described. Parasites attained an average weight of 7 mg and occupied the posteriad positions within the fish intestine (77%). Parasites from this sample of Irish brown trout attained a similar average size to those found in chub and barbel from England. 42.3% of the total parasites examined contained ovarian balls only and 17% contained fully mature acanthors. Therefore only a moderate proportion of female worms contained mature acanthors in these trout whereas the majority of worms recovered from a sample of chub were gravid. Utilizing a logistic regression model, parasite size, season, and site of host capture emerged as particularly significant factors which contribute to whether a parasite contains mature eggs.