The foraging behaviour of Brünnich's guillemots Uria lomvia was studied in a high-arctic fjord system (Kongsfjorden) in western Spitsbergen. The physical oceanographic characteristics of the water change from the head of the fjord and westward into the Greenland Sea, and are reflected in the vertical profiles of water temperatures in different parts of the area. Nine chick-rearing Brünnich's guillemots were instrumented with temperature-depth recorders, which generated vertical temperature profiles of the dives. These were compared to synoptic measurements of the water temperature characteristics of the region. This method was used to locate the foraging areas of the Brünnich's guillemots and to study the foraging site fidelity of individual birds. The results showed that only three of the nine birds foraged outside Kongsfjorden during the study period, and only 26 of the 186 dive bouts (14%) were conducted outside the fjord, 48–58 km from the colony. Most dives were probably made only a few kilometres from the colony. The data indicate that the birds showed strong fidelity to foraging areas at spatial scales of 1–20 km. However, the birds sometimes moved between feeding areas characterized by different vertical temperature profiles. The guillemots made 2229 dives during the study period and spent c. 10% of their time under water. The diving depth averaged 45 m, and the dive duration averaged 97 s. The deepest dive recorded was 136 m and lasted 196 s. We did not find any diel rhythm in the diving depths of the Brünnich's guillemots. Also, we found no diel pattern in diving frequency. These findings contradict the predominance of night-time diving observed in studies of guillemots undertaken further south.