The ability of a grey seal, Halichoerus grypus, to navigate a variety of obstacle courses was examined under light and dark conditions in a 6.1 m diameter tank. Four maze patterns were constructed by varying the positions of 18 vertical rods in the tank. One hundred trials were conducted, each consisting of a 2 minute light phase followed by a 2 minute dark phase. The order of presentation of the maze patterns was randomly determined. All trials were monitored for sound. A phosphorescent tag was attached to the seal to allow monitoring of its movements in the dark. The mazes differentially modified the seal's path of travel. Average rates of movement were determined under four conditions: light with no maze present - 108 cm/sec; light with a maze present - 65 cm/sec; dark with no maze present - 15 cm/sec; and, dark with a maze present - 14 crn/sec. The seal displaced 7 obstacles in the light and 17 in the dark. Trials in the dark with errors had a significantly higher mean distance traveled than those without errors: x = 21.6 m vs x = 16.2 m, p<.05. Clicks, clusters of clicks, and hisses were recorded, mostly in the dark. There was a negative correlation between sound production and improved navigational ability. It is suggested the seal relied upon visual orientation in the light, and on tactile cues provided by the vibrissae coupled with memory in the dark.