Penguins may exhibit plasticity in their diving and foraging behaviors in response to changes in prey availability. Chinstrap penguins are dependent predators of Antarctic krill in the Scotia Sea region. Both the sizes of krill found in penguin diets and acoustic estimates of krill biomass have fluctuated in recent years. We therefore examined the diet of chinstrap penguins at Livingston Island, South Shetland Islands in relation to their diving and foraging behavior using time-depth recorders (TDRs) over five seasons: 2002-06. When krill were smaller, chinstrap penguins often exhibited a shift to deep dives after sundown, and then resumed their shallower pattern at sunrise. These nighttime dives were unexpectedly deep (up to 110m) and mean nighttime depths sometimes exceeded those from the daytime. The average annual size of krill was negatively correlated to the number of penguins foraging on fish, mean nighttime dive depths, and the proportion of foraging trips occurring overnight. Based on these patterns, we suggest that when krill were small, penguins foraged more on myctophid fish. The average krill size was negatively correlated to the time chinstrap penguins spent foraging which suggests that penguins made this switch to fish with a cost: more time was spent at sea foraging.