B chromosomes (Bs) were detected in a diploid guineagrass (2n=16) strain derived from seeds collected in Tanzania. The number of Bs was constant at the metaphase I (MI) of pollen mother cells in the same plant, and the size of the B in the bivalents and univalents was approximately the same as that of the normal (A) chromosomes at MI. The Bs did not pair with the A chromosomes, but frequently paired with one another when two or more were present. Although the Bs were transmitted through both male and female gametes, male gametes transmitted twice the number of Bs received through microspores after meiosis. Bs had no significant effect on the length of the panicle and leaf, but plant height was decreased by these chromosomes. Pollen and seed fertility was severely depressed in plants having more than three Bs.