Spontaneous DNA insertions from supernumerary B chromosomes (Bs) into the standard A chromosome complement were detected in rye (Secale cereale L.), using fluorescent in situ hybridization (FISH) analysis with the D1100 B-specific sequence probe. The insertions were seen in individuals derived from plants possessing deleted Bs, characterized in this study by not having the B-specific sequences that are normally found at the distal part of the long arm of the standard rye B. This result supports the case for the spontaneous introgression of B-specific DNA into the A chromosome genome, and it indicates that 'cross-talking' between A and B chromosomes may occur in wild populations.