(P34) The order Aulopiformes consists of 13 families, in which 42 genera are composed of 219 species. This order has a characteristic feature that it contains both synchronous hermaphroditic and gonochoristic species; the former tends to inhabit deep-sea and the latter inhabits shallow waters. In the morphological and ecological studies, Smith (1975) proposed his idea that a gonochoristic group of species, which is minor in this order, was evolutionarily derived from hermaphroditic species. To examine Smith's proposal, we conducted molecular phylogenetic and cytogenetic studies of those species. It provides us with a unique opportunity to elucidate the evolutionary process of sex. First, we conducted a phylogenetic analysis by sequencing the complete mitochondrial cytochrome b genes for two synchronous hermaphroditic species (Chlorophthalmus albatrossis and Chlorophthalmus sp.) and four gonochoristic species (Saurida elongata, Synodos ulae , Synodus hoshinonis, and Trachinocephalus myops). We also examined karyotypes for one hermaphroditic and three gonochoristic species. As a result, the phylogenetic tree obtained showed that the hermaphroditic species diverged from a group of the gonochoristic species, on the contrary to Smith's idea. Second, we found that in gonochoristic species, heteromorphic sex chromosomes were clearly distinguishable from autosomes under a microscope, although sex chromosomes of almost all of teleost fishes, in general, difficult to be detected by the microscopic analysis. On the other hand, our preliminary analysis indicated that the hermaphroditic species has no heteromorphic chromosome. Thus, we concluded that the hermaphroditic species were evolutionarily derived from the gonochoristic species by losing the heteromorphic sex chromosomes during evolution. Taking into account Ohno (1967)'s suggestion that heteromorphic sex chromosomes emerged from autosomes, we propose the hypothesis that heteromorphic sex chromosomes have undergone repeated events of appearance and disappearance in the evolutionary course of fish.