To elucidate possible mechanisms for maintaining B-chromosomes (Bs) in natural populations of Aegilops mutica, cytological behavior of Bs during first pollen grain mitosis was examined in five plants with 1 to 3Bs. Male and female transmission rates of Bs were also studied in the progeny obtained from reciprocal crosses between the plants with and without Bs. In 1B plants, 44% of the PMCs lost the B during meiosis. In 2B and 3B plants, only 10% and 20% of the Bs were lost during meiosis, respectively. No difference was observed in the frequency of B-carrying progeny between the reciprocal crosses, suggesting that the rates of B-chromosome loss during meiosis were similar in EMCs and PMCs. At the anaphase and telophase of first pollen grain mitosis, two sister chromatids of Bs underwent nondisjunction and preferential distribution into the generative nucleus in almost all the B-carrying pollen grains. As a result, most of the progeny obtained from the crosses using B-carrying plants as male parents had even numbers of Bs. No such accumulation of Bs occurred in the course of megasporogenesis, judging from the female transmission rate of Bs. Further, it was suggested that B-carrying gametes took part in fertilization without any advantages or disadvantages. It was, thus, concluded that nondisjunction and preferential distribution of B-chromatids into the generative nucleus during first pollen grain mitosis in 2B and 3B plants were the most effective accumulation mechanisms of Bs in Ae. mutica. The frequency of Bs in natural populations of Ae. mutica might be determined by their accumulation during the first pollen grain mitosis in 2B and 3B plants, the loss during meiosis in 1B plants, and the loss caused by the reduced viability and/or fertility in individuals with a larger number of Bs.