Division of labor is fundamental to the success of all societies. The most striking examples are the physically polymorphic worker castes in social insects with clear morphological adaptations to different roles. These polymorphic worker castes have previously been thought to be a classic example of nongentically controlled polymorphism, being mediated entirely by environmental cues. Here we show that worker caste development in the leaf-cutting ant Acromyrmex echinatior has a significant genetic component. Individuals of different patrilines within the same colony differ in their propensities to develop into minor or major workers. The mechanism appears to be plastic, with caste destiny resulting from interplay between nurture and nature. Unlike the few other recently discovered examples of a genetic influence on caste determination, the present result does not relate to any rare or exceptional circumstances, such as interspecific hybridization. The results suggest that a significant role of genetics may have been overlooked in our understanding of other complex polymorphisms of social insects.