The feeding behavior of antarctic knll Euphausia superba on ice algae was observed in situ and in the laboratory. Field observations by divers confirm that knll uLhze natural sea ice microalgae for food. Laboratory investigations show that melting ice releases algae into the water column which induces area-intensive foraging behavior in M1. This behavior is characterized by high speed swimming and rapid turning, accompanied by rapid opening and closing of the thoracic appendages, also known as the feedmg basket. Presentation of increased concentrations of ice algae to laboratory populations of knll significantly increased euphausiid responsiveness which led to location of and direct grazing upon the undersurfaces of ice containing microalgae. Foraging behavior of krill on ice algae appears to be affected by the spatial patchiness of the algae withln the ice and on the rate of algal cell release from ice. We propose that sea ice algae is an abundant and predictable food resource for knll during austral winters, when phytoplankton food resources are depleted.