This paper describes the diet and foraging behaviour of Ad6lie penguins at B6chervaise Island during 1998-99, a season of high chick mortality. One thousand nests perished in the three weeks following first hatch, when the chicks were very small. A total of 802 chicks reached crèche age from 1880 nests compared to previous "good" seasons in which between 1200 and 1800 chicks crèched on the island. Evidence from analyses of foraging location, foraging trip duration and diet led to the conclusion that the death of chicks during the guard stage resulted from decreased feeding frequency due to adult birds spending longer at sea foraging than in normal years. In most previous seasons birds have foraged both at the continental shelf edge (particularly females) as well as locally (particularly males). This season male penguins carried out fewer local trips, and both sexes spent longer at sea than in "good" years. Meal masses brought back to the chicks appeared normal, but feeding frequencies were reduced. These findings contrast with observations in 1994-95 Ca season in which all chicks starved to death) when smaller meals were delivered and birds foraged further offshore than in this or any other season studied. The significance to CEMP of these variations in foraging behaviour is discussed.