Trivalent organoarsenicals such as methylarsenite (MAs(III)) are considerably more toxic than inorganic arsenate (As(V)) or arsenite (As(III)). In microbial communities MAs(III) exhibits significant antimicrobial activity. Although MAs(III) and other organoarsenicals contribute to the global arsenic biogeocycle, how they exert antibiotic-like properties is largely unknown. To identify possible targets of MAs(III), a genomic library of the gram-negative bacterium, Shewanella putrefaciens 200, was expressed in Escherichia coli with selection for MAs(III) resistance. One clone contained the S. putrefaciens murA gene (SpmurA), which catalyzes the first committed step in peptidoglycan biosynthesis. Overexpression of SpmurA conferred MAs(III) resistance to E. coli. Purified SpMurA was inhibited by MAs(III), phenylarsenite (PhAs(III)) or the phosphonate antibiotic fosfomycin but not by inorganic As(III). Fosfomycin inhibits MurA by binding to a conserved residue that corresponds to Cys117 in SpMurA. A C117D mutant was resistant to fosfomycin but remained sensitive to MAs(III), indicating that the two compounds have different mechanisms of action. New inhibitors of peptidoglycan biosynthesis are highly sought after as antimicrobial drugs, and organoarsenicals represent a new area for the development of novel compounds for combating the threat of antibiotic resistance.