SUMMARYA genetic map of eight structural genes involved in arginine synthesis in Proteus mirabilis strain 13 was obtained by transduction. Genes argE, C, B, G, H form a cluster linked to the gene cysE, the argD gene is linked to a locus which controls resistance to I mg./ml. streptomycin, argF is linked to a pyrimidine marker, while the argA gene could not be co-transduced with other markers. The clustering and order of argE, C, B, H, the linkage of argD to str-r and the non-linkage of argF and argA to other arginine genes are features shared by P. mirabilis, Escherichia coli and Salmonella typhimurium. Proteus mirabilis differs from the other organisms in that argG is included in the argE, C, B, G, H cluster. This cluster is closely linked to cysE and not to methionine markers. ArgF, which is the gene for the sixth step in the pathway, is linked to a pyrimidine marker while in E. coli this step is governed by two genes, one of which (argI) is linked to pyrB.