ABSTRACT: Chick fledging body mass of the common guillemots Uria aalge breeding at Stora Karlsö in the Baltic Sea between 1989 and 2000 were studied in order to reveal any effects of an increased abundance of their main prey species, spratSprattus sprattus. Chick fledging body mass has decreased during the study period, and so has the condition of sprat. The deteriorated condition of the prey is most probably responsible for the observed decreased fledging body mass in guillemotchicks. This study shows that the condition of fish preyed upon by seabirds, in addition to availability, may be an important factor when studying seabird-fish interactions. It is clearly indicated that changes in the sprat population, as a consequence ofchanges in the Baltic Sea, have affected common guillemot fledging body mass.