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The evolution of male secondary sexual traits traditionally has been ascribed to precopulatory sexual selection. In contrast, the importance of postcopulatory sexual selection for the evolution of secondary sexual traits is uncertain, and what little evidence exists for this process to contribute to the evolution of such traits is mixed. Here we test the hypothesis in Drosophila bipectinata Duda that the male sex comb, a rapidly evolving secondary sexual trait, is under positive postcopulatory sexual selection. We extracted replicate genetic lines exhibiting relatively large and small sex comb size from a natural population. Males from these lines were subjected to an assay of competitive fertilization ability, measured as P2, the proportion of a females clutch of eggs fertilized by the second male to mate. Males with the largest sex combs sired more offspring than less ornamented individuals, demonstrating for the first time in any Drosophila species that postcopulatory sexual selection favors increasing sex comb size. This study identifies a postcopulatory selective mechanism that may be contributing to the evolutionary diversification of a secondary sexual trait. [ABSTRACT FROM AUTHOR]