Gavia immer (Common Loon) utilizes freshwater lakes for nesting and breeding, but winters off both US coasts and the Gulf of Mexico in marine habitats. They are primarily piscivorous. In the winter, Common Loons feed predominately as single individuals, but they occasionally form small groups or flocks. The groups likely facilitate greater feeding efficiency. The largest previously reported foraging aggregation of wintering Common Loons was approximately 200 individuals. We report an extremely large wintering flock of >600 loons foraging 17 km from the southern coast of Mississippi. More work is needed to understand the prevalence of larger groups of overwintering loons as well as temporal and spatial factors that might predict their occurrence.