The mangrove killifish, Kryptolebias marmoratus and another related killifish species are only two known self-fertilising vertebrates. By making use of the self-fertilising ability, it is possible to isolate both zygotic and maternal mutant lines from this fish with one generation earlier than zebrafish and medaka allowing, much smaller scale and shorter process of mutant screening and maintenance. We have isolated two mutant lines that shows abnormalities in the tail development, R109 (short-tail/stl) and R228 (ball-tail/btl). At the somitogenesis stage, both mutants show normal length of the tail, but in the stl mutant, gene expression in the tail including sox3 (neural tube), col9a1b (notochord) and hsp90 (muscle) all reduce. The tail stop growing at late somitogenesis and regress. In the btl mutant, sox3 and col9a1b expression is normal but hsp90 is strongly suppressed suggesting the defect is specific to the tail somite muscle. We are currently mapping these mutations in the genome to identify the mutations and will report the progress of the mutation analyses in the meeting.