Stomach contents from 53 Common Guillemots Uria aalge beached at the Island of Helgoland in the southeastern North Sea were examined for prey remains. In winter 2000/2001, the prey spectrum was quite diverse. Remains of species belonging to ten different families of teleost fishes were found, with pipefishes, gobies, sandeels and clupeids being the most abundant prey. Invertebrates contributed only 1 % of all prey items. The diversity was considerably smaller in winter 2001/2002, when clupeids and sandeels had the highest numerical abundance and only three other families were found. The number of sandeels and clupeids in the stomachs might be connected with water temperature. When these fish families were present in the stomachs, the water temperature on the day before collecting the dead Guillemots was significantly higher than when these fish were absent in both winter periods. The few samples collected in summer contained mainly sandeels and clupeids, fish species which are also brought to the colony for display and to feed the chicks. However, the fishes found in the stomachs of the adult birds were smaller than fishes carried to the breeding ledges. Additionally, a dragonet and a cephalopod were found in the stomachs, prey that have never been observed in the colony. This confirms our assumption that observations of the fishes brought to the colony are not representative for the diet of adults. Adult Guillemots deliver relatively large fishes of high caloric density to the chicks. During self-feeding, they are much more opportunistic and also consume smaller and leaner prey. This is in accordance with Central Place Foraging Theory. Difficulties in the methods employed and the effect of oiling on diet composition are also discussed in this study. White oiling seemed to have no influence on the total number of prey items found in the stomachs of the dead Guillemots, we found sandeels and gobies more frequently in oiled and pipefishes more often in unoiled birds.