To evaluate the gene action and interaction of quantitative trait loci (QTLs) as a single Mendelian factor, attempts were made in this study to isolate individual QTLs causing long grain in a large- grain rice cultivar ‘BG 1’, in other words, to establish a series of nearisogenic lines which had long-grain alleles derived from BG 1 at only one QTL and the short0grain alleles at the other QTLs. In the course of developing recombinant inbred lines from the cross between BG 1 and a short-grain cultivar Koshihikari, the lines were surveyed through backcrossing of the recombinant inbreds and by progeny test. Until the BC2F2 generation, a total of 8 lines, which obviously showed monogenic segregation within the respective progenies in which a single QTL was involved, were obtained. All of the long-grain alleles at the isolated QTLs were recessive to the short-grain alleles. The process of isolation of 2 different QTLs was described in detail as an example of the strategy adopted in this study. These isolines for isolated QTLs should contribute significantly not only to the characterization of individual QTLs and their combinations but also breeding programs as gene sources to be used directly.