We have developed from crosses of oat (Avena sativa L.) and maize (Zea mays L.) 50 fertile lines that are disomic additions of individual maize chromosomes 1-9 and chromosome 10 as a short-arm telosome. The whole chromosome 10 addition is available only in haploid oat background. Most of the maize chromosome disomic addition lines have regular transmission; however, chromosome 5 showed diminished paternal transmission, and chromosome 10 is transmitted to offspring only as a short-arm telosome. To further dissect the maize genome, we irradiated monosomic additions with gamma rays and recovered radiation hybrid (RH) lines providing low- to medium-resolution mapping for most of the maize chromosomes. For maize chromosome 1, mapping 45 simple-sequence repeat markers delineated 10 groups of RH plants reflecting different chromosome breaks. The present chromosome 1 RH panel dissects this chromosome into eight physical segments defined by the 10 groups of RH lines. Genomic in situ hybridization revealed the physical size of a distal region, which is represented by six of the eight physical segments, as being approximately 20% of the length of the short arm, representing approximately one-third of the genetic chromosome 1 map. The distal approximately 20% of the physical length of the long arm of maize chromosome 1 is represented by a single group of RH lines that spans >23% of the total genetic map. These oat-maize RH lines provide valuable tools for physical mapping of the complex highly duplicated maize genome and for unique studies of inter-specific gene interactions.