What evolutionary processes promote species diversification is a fundamental question in biology. The ricefishes are highly diversified on the island of Sulawesi. Dynamic geological changes of the island are considered to have led to geographical isolations of populations and eventual allopatric speciation. However, recent studies have revealed that introgressions had occurred in independent lineages in separate places and times. Furthermore, the introgressions in the ancestors of sympatric species are supposed to have possibly promoted the speciations. These make Sulawesian ricefishes an intriguing system to study the roles of hybridisation in organismal diversification. Lake Poso harbours three species of the ricefish of Oryzias, which are inferred to have speciated in sympatry within the lake. The three species are isolated by resource partitioning and assortative mating. Despite the limited conditions of speciation in sympatry by theory, there are cases of sympatric speciation known from nature. Mechanisms that facilitate sympatric speciation are, however, not well understood. By revealing evolutionary processes that shaped the isolating barriers and the underlying genetic bases in Lake Poso ricefish, we expect to elucidate genetic mechanisms of sympatric speciation and potential contributions of hybridisation to the speciation in the presence of gene flow.