From January 1995 to January 1996 and in April 1996, 9026 specimens of herring Clupea haren-gus membras were caught in ICES-Subdivisions 24, 25, 26 and 27 in the southern Baltic. All fish were examined for nematodes. 5242 Anisakis simplex (Rudolphi, 1809) and 3 specimens of Con-tracaecum osculatum (Rudolphi, 1809) (3rd stage larvae) were collected. Only coastal spring herring were infected with A. simplex; C. osculatum was found in open sea herring. The total body condition factor K of each fish was calculated according to Fulton. The mean body condition factor was found to be higher in infected than in non-infected fish. The highest value of factor K was observed in fish with gonads at stage 6. Factor K reached a minimum in fish in-fected with 11-20 specimens of A. simplex larvae.