The diving behavior of 7 chinstrap penguins (Pygoscelis antarctica) (N=12,171 dives) was measured concurrently with a hydroacoustic assessment of the vertical distribution and abundance of their primary prey, krill (Euphausia superba) in the vicinity of Seal Island, South Shetland Islands, Antarctica between January 19 and March 10 1992. Krill was found to show a distinct diel migration pattern, being dispersed in the upper portion of the water column at night and more concentrated and deeper during the day. Penguin foraging effort was found to be concentrated around noon and midnight, with a reduction in effort around dawn and dusk (local apparent time). The mean and maximum depth of chinstrap penguin dives was found to follow the diel migration pattern of the krill. On average, chinstrap penguins dove to the shallow limit of the distribution of krill. The maximum depth of penguin dives did not exceed the maximum depth distribution of krill. These patterns in diving behavior may result from diel changes in the methods used by penguins to locate and capture prey. Our results suggest that penguins do not require extremely dense aggregations of prey in order to successfully capture sufficient krill to meet their energetic needs. We hypothesize that the diel migration pattern found in krill which has been found to be variable in different study locations at different times may in part be determined by the intensity of predation pressure by predators which feed in the upper portion of the water column (i.e. seabirds and marine mammals).