Smallholder farmers rely on labor exchange to generate agricultural work when cash is rare and credit unavailable. Reciprocal altruism, biased by genetic kinship, has been implicated as the mechanism responsible for labor exchange; however, few empirical tests confirm this proposition. Competitive altruism could be operating if people differ in ability and use this information as a criterion for partnership selection. Labor exchange data are presented from a Dominican smallholder village over a 10-month period within the village’s primary cash economic opportunity, bay oil production. Results indicate that competitive altruism better explains variation in labor exchange relationships and group size than reciprocal altruism and kinship, suggesting the presence of a biologic market for male exchange relationships. Bay oil laborers vary in altruistic behaviors, causing reputations for altruism to emerge. Men with reputations as high-quality altruists generate larger labor groups in bay oil production than do poor-quality ones. Larger groups induce bargaining wars, causing men to compete through altruistic acts, which allows high-quality individuals to discriminate potential partners for labor exchange relationships. Men with better reputations achieve more same-sex reciprocal partnerships but not a greater incidence of conjugal partnership, suggesting that male altruism is intra- but not intersexually selected.