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Low-temperature-sensitive steriUty has become one of the major obstacles in indica-japonica hybrid rice breeding. The objectives of this paper were to evaluate the extent of the fertility reduction and to determine the genetic basis of low-temperature-sensitive sterility. Seventeen varieties were crossed in various ways to produce 21 F|S including 16 indica-japonica hybrids. Fertility of the F]S and their parents was examined under both high and low temperature conditions. Considerable reduction in spikelet fertility was observed under low-temperature conditions in the majority of the indica-japonica hybrids having at least one wide compatibility parent. However, the extent of fertility reduction varied greatly, depending on the parental genotypes. Data from five pairs of reciprocal crosses indicated that the cytoplasm had no effect on fertility reduction. The more-or-less bimodal distribution of the fertihty segregation of one BCiF, and two F^ populations under low-temperature conditions suggested that the lowtemperature-sensitive sterility was controlled by only one or a few genes. It was also shown that the low-temperature-sensitivity is not related to wide compatibility. We conclude that it is possible to develop indicajaponica hybrids with wide compatibility and also insensitivity to the low-temperature conditions.