Matano Ricefish (Oryzias matanensis Aurich, 1935) is an endemic freshwater fish with economic value. This study aims at determining the sex ratio and first maturity size of Matano ricefish. The study was conducted from November 2017 to October 2018 in Lake Towuti, South Sulawesi. Sample was held analysis the Fisheries Biology Laboratory, Department of Fisheries, Hasanuddin University. Total number of fish obtained during the study was 880 individu, consist of 410 male fish and 470 female fish. Sex ratio was analyzed with the chi-square test, while the average size at first gonad maturity was estimated using the Spearman-Karber method. The results showed that the sex ratio of male and female medaka ricefish was 1.00: 1.15, while size at first maturity of male gonad was 36.46 mm, and the female was 34.37 mm.