Introgression of genes in species crosses can be observed morphologically in backcrossed or selfed progenies, but the phenotype does not give information about the movement of DNAs. Cytogenetic markers allow for visualization of specific DNAs in a genome. Few cytogenetic markers are available in onion to monitor the introgression of DNA in species crosses. Genomic in situ hybridization (GISH) provides a way to locate unique DNA sequences contributed by parents. We are using GISH to monitor the movement of DNAs from A. fistulosum into A. cepa. Results of experiments using A. fistulosum as probe DNA, and A. cepa as blocking DNA will be reported. Also presented are hybridization sites observed in F1BC3 progeny of the GISH.