A total of 112 flounder, Platichthys flesus, was surveyed for the presence of Pomphorhynchus laevis (Acanthocephala). No acanthocephalans were found in 35 fish, 40 fish were infected with 1–5 thorny-headed worms, and in 37 fish the intensity of infection exceeded 6. The fish surveyed were also studied for the composition of the resident microflora of the terminal portion of the intestine—the characteristic site for P. laevis. It turned out the number of worms was positively correlated with the bacterial count. The bacteria cultured from the material sampled from intestines of infected- and non-infected fish exhibited similar specific composition and the differences were only in the intensity of individual species. The most frequently isolated bacteria, represented the genera: Aeromonas, Pseudomonas and Shewanella. The least frequent were species of Chromobacterium, Escherichia, Providencia and Serratia. The presently reported study results, suggest that bacteria species, constituting physiological bacterial flora, may become, under certain conditions, potentially pathogenic, through substantial increase in their abundance. The factor responsible for disturbing the natural balance in the intestine may be the thorny-headed worm, Pomphorhynchus laevis.