Cytogenetic maps of sorghum chromosomes 3-7, 9, and 10 were constructed on the basis of the fluorescence in situ hybridization (FISH) of 18-30 BAC probes mapped across each of these chromosomes. Distal regions of euchromatin and pericentromeric regions of heterochromatin were delimited for all 10 sorghum chromosomes and their DNA content quantified. Euchromatic DNA spans 50% of the sorghum genome, ranging from 60% of chromosome 1 (SBI-01) to 33% of chromosome 7 (SBI-07). This portion of the sorghum genome is predicted to encode 70% of the sorghum genes (1 gene model/12.3 kbp), assuming that rice and sorghum encode a similar number of genes. Heterochromatin spans 411 Mbp of the sorghum genome, a region characterized by a 34-fold lower rate of recombination and 3-fold lower gene density compared to euchromatic DNA. The sorghum and rice genomes exhibit a high degree of macrocolinearity; however, the sorghum genome is 2-fold larger than the rice genome. The distal euchromatic regions of sorghum chromosomes 3-7 and 10 are 1.8-fold larger overall and exhibit an 1.5-fold lower average rate of recombination than the colinear regions of the homeologous rice chromosomes. By contrast, the pericentromeric heterochromatic regions of these chromosomes are on average 3.6-fold larger in sorghum and recombination is suppressed 15-fold compared to the colinear regions of rice chromosomes.