Medaka (Oryzias latipes) is widely used in research in the fields of biology, medicine, environmental science and fisheries. Zebrafish and medaka are well established as genetic model systems in which large-scale mutagenesis has been successfully performed, and for which EST data, BAC libraries, and fine linkage maps have been accumulated. Among rayfinned fish, there is a large evolutionary distance between medaka and zebrafish. In contrast, the evolutionary distance between medaka and two species of pufferfish, fugu (Takifugu rubripes), and tetraodon (Tetraodon nigroviridis), is almost comparable to that between humans and rodents, and the current genome project is showing that their genome organization is well conserved. Comparison of genome structure among teleosts and mammals helps our understanding of the orthologous gene structure and the evolution of gene families in vertebrates. In addition, gene functions have to be analyzed by both forward and reverse genetics. The Targeting Induced Local Lesions IN Genome (TILLING) system, which includes random mutagenesis, followed by screening for induced mutations in the target genes, is a powerful tool for studying the functional genomics of both medaka and zebrafish.