Sorghum bicolor (L.) Moench (sorghum) is a naturally well-established diploid species with 2n = 2x = 20. Here we provide evidence to identify the genomic relationships of S. bicolor and its extant relatives using genomic in situ hybridization. The genomic divergences between S. bicolor and its close relatives may be presented as: S. bicolor-S. x drummondii (Nees ex Steud.) Millsp. & Chase < S. bicolor-S. x almum Parodi < S. bicolor S. arundinaceum (Desv.) Stapf < S. bicolor-S. propinquum (Kunth) Hitchc. Sorghum arundinaceum probe signals showed spotted or painted patterns on the S. bicolor chromosomes, indicating that the degree of genomic divergence between S. bicolor and S. arundinaceum is distinct. It is reasonable to infer that S. bicolor might have diverged from S. arundinaceum in the early diversification history of the subgenus. The probe signal intensity of S. arundinaceum and S. propinquum is greater on the 17 chromosomes than on the remaining 23 chromosomes of S. halepense (L.) Pers., whereas S. bicolor and S. propinquum probes produced spotted or patchy signals on the 40 S. halepense chromosomes. Therefore, the degree of genomic divergence may be presented as: S. halepense-S. bicolor < S. halepense-S. arundinaceum < S. halepense-S. propinquum.