The suggested use of decommissioned oil platforms as artificial reefs initiated a study of the abundance and residence of fish in the vicinity of the Albuskjell 2/4 Fox platform in the Norwegian sector of the North Sea. A study of fish residence was carried out by means of cylindrical coded ultrasonic VEMCO transmitters surgically implanted into the abdominal cavity of fish caught by handline and pots near the platform. After the operation, the fish were kept in a tank of running seawater to ensure that only fully recovered fish were released. Twenty-nine cod and two haddock were tagged. Eight ultrasonic receivers anchored around the platform recorded the presence of tagged fish. An additional receiver was placed at a neighbouring platform 8 km away. Data were collected from 25 May to 13 August 1998 and showed that approximately 50% of the tagged fish remained at or near the platform throughout this period. Four tagged cod were registered by the receiver at the neighbouring platform. Five tagged cod have been reported caught, one at the study platform, two at a nearby platform, and two at 9 and 150 km distance, respectively. When the platform was revisited in May 1999 four cod were detected.