Occurrence of Pomphorhynchus laevis (Muller, 1776) in cage-reared rainbow trout Oncorhynchus mykiss (Walbaum, 1792) and the effects of parasitism on the fish condition were studied between July and November 2014 in Isikli Spring. A total of 221 O. mykiss specimens were examined. In addition to P. laevis, three other parasite species; Trichodina sp., Ichthyophthirius multifiliis Fouquet, 1876 and Diplostomum sp. were also recorded. P. laevis was numerically the predominant species with the highest prevalence (77.8%), mean intensity (6.74), and mean abundance (5.24). In total 1160 P. laevis specimens were collected. The mean abundance of P. laevis increased over a period of months from 2.5 individual parasites per fish in July to 7.1 in November. The mean intensity of P. laevis increased with increasing fish host length from 1.5 in the 5.0-6.9 cm length class to 18.4 in the 25.0-26.9 cm length class. Although there was no statistically significant bias in the spatial distribution of P. laevis within the gut (p > 0.05), the parasite tended to prefer the pyloric caeca (47.9%). The larval stage (cystacanth) of the acanthocephalan parasite were obtained from the haemocoel of Gammarus obnixus Karaman and Pinkster, 1977. The value of Fulton's condition factor (K) ranged from 0.84 to 1.66.