Quantitative studies of predator-prey interactions depend on a knowledge of their spatial dynamics and behaviour. Studies on marine vertebrates have hitherto been precluded by the difficulty of acquiring the relevant data. (2) Continuous records of diving depths of female Antarctic fur seals on 3-8 day feeding trips to sea from South Georgia were analysed in conjunction with data on diel changes in the abundance and distribution of their main prey, krill. (3) In 36 complete days foraging by seven seals, 75% of 4273 dives were at night. Dives then were consistently shallower (dive depth <30 m) than in daytime (mostly 40-75 m). (4) This closely matched changes in the vertical distribution of krill, nearly all of which was below a depth of 50 m from 09.00-15.00 h, with substantial quantities above 40 m only between 21.00-06.00 h. (5) Although over 40% of krill in the water column at any time of day was below 75 m, only 3% of dives exceeded this depth. We suggest that because krill migrate vertically fur seals are able to exploit them most efficiently during shallow dives at night.