Background <p>The accumulation of sexually antagonistic mutations on chromosomes has been favored in driving the evolution of sex chromosomes. However, empirical evidence has been limited to species with heteromorphic sex chromosomes, where the patterns of gene expression on sex chromosomes differ drastically from those on autosomes. It is little known if the sexualization of gene expression would have started on homomorphic sex chromosomes.</p> Results <p>To investigate the evolutionary patterns of sex-biased expression, we examined four species of spiny frogs, using high-throughput RNA sequencing for gonads and sex-specific tissues. We find little support for both the enrichment of sex-linked genes and the excess of sex-linked gene expression. Additionally, there is no evidence supporting for sex-biased gene expression near the candidate sex-determining locus. We find no significant accumulation of sex-biased genes on sex chromosomes. Further, the homomorphic sex chromosomes do not show evidence of a faster rate of evolution.</p> Conclusions <p>The sexualization of sex chromosomes in species with homomorphic sex chromosomes may not be as indispensable as hypothesized.</p>