Courtship song serves as a sexual signal and may contribute to reproductive isolation between closely related species. Using lines of Drosophila pseudoobscura experimentally selected under different sexual selection regimes, we tested whether increased promiscuity and enforced monogamy led to evolutionary changes in courtship song elements. In D. pseudoobscura, males produce both a low and high rate repetition song. We found that both song types diverged after selection and that the direction of changes was consistent with ordered hypotheses of predicted directions of change under the different mating system structures. In particular, latency to the initiation of song and duration of the interpulse interval (IPI) decreased in highly promiscuous lines and increased in monogamous lines. These results suggest that courtship song may rapidly evolve under different mating system structures, representing either functional evolution of more stimulatory song under conditions of strong sexual selection, or might result from increased courtship vigor, or represent correlated evolution. Some speciation theory predicts that increased sexual selection should result in increased variance of traits between allopatric populations, facilitating reproductive isolation. We also found that courtship song elements were not equally variable between replicate lines.