Species often produce sterile hybrids early in their evolutionary divergence, and some evidence suggests that hybrid sterility may be associated with deviations or disruptions in gene expression. In support of this idea, many studies have shown that a high proportion of male-biased genes are underexpressed compared to non-sex-biased genes in sterile F1 male hybrids of Drosophila species. In this study, we examined and compared patterns of misexpression in F1 hybrid male third instar larvae of Drosophila simulans and its sibling species, D. mauritiana. We analyzed hybrids using custom cDNA arrays we developed from RT-PCRs of spermatogenesis-related transcripts from these species and another sibling species (D. sechellia). The results from a commercial genome-wide array and custom chip for adults of this species pair, from the custom chip and the genome-wide chip for adults of the D. simulans-D. sechellia species pair, and from the larvae of the D. simulans-D. sechellia species pair, are presented separately. Keywords: Comparison of pure-species Drosophila expression to hybrid expression Overall design: We analyzed 11 arrays: 4 of sim-mau vs. mau, 4 of sim-mau vs. sim, and one each of self-hybridizations (sim vs. sim, sim-mau vs. sim-mau, mau vs. mau).