A comparative parasitology study of gobiid fishes from two sites on the western Baltic Sea was done to get information on the quality of the respective environments. The parameters used were the infracommunity, the species numbers and abundance with respect to the size classes of the hosts, and the abundance with respect to the seasons. The hosts, Gobius niger, Gobiusculus flavescens, Pomatoschistus minutus, and P. pictus, were caught at Blank Eck (Kiel Bight) and Dahmeshöved (Lübeck Bight) during 1992 and 1993. The infracommunities of Dahmeshöved attained higher rates of prevalence of helminth parasites than did those of Blank Eck, as was valid in all host specimens harboring one or two parasite species. The group bearing three parasite species was present in Dahmeshöved but absent in Blank Eck. Parasite species numbers, which were similar in the two sites, increased from the smallest to the largest hosts but changed with growth. With one exception, this result is also valid for the abundance, which clearly differed between the hosts and between the sites. Whereas species numbers as well as abundance rates were low in spring, they increased in summer. The results reveal that the composition of infracommunities depends on the local parasite fauna and on the strength of general host defense mechanisms, whereas that of the component community is dependent on the susceptibility of hosts to single parasite species and on the density of intermediate parasite stages. This is also the reason for the stronger infestation of hosts from Dahmeshöved as opposed to those from Blank Eck, where the degree of eutrophication is lower and, therefore, fewer herbivorous intermediate hosts can exist.