The B chromosomes (Bs) of rye (Secale cereale) have been studied at interphase in terms of their chromatin organization, patterns of nuclear disposition and physical association in plants with two, three, and four Bs. The study was made in the Lindström strain of hexaploid wheat, which carries the rye Bs as an addition line, by in situ hybridization with a B-specific probe and by genomic in situ hybridization (GISH) with rye genomic DNA, enabling whole chromosome painting. Repetitive sequences common to the As and Bs of rye allow for visualization of the rye B at interphase in the wheat background. A B-specific probe enables the orientation of two or more Bs to be determined, and the combination of both probes used together gives information on the disposition of the Bs and on their patterns of physical association within the nucleus. The Bs form linear "strings', and the ends of their long arms, which can be detected by the B-specific probe, are usually located within the hemisphere of the nucleus that has the least condensed chromatin. There is dose-dependent association, and even numbers (2B, 4B) have a greater preference for association than odd ones (3B).