To investigate azithromycin susceptibility in Shigella sonnei in the United States, we examined the azithromycin minimum inhibitory concentrations (MICs) of outbreak and routine human S. sonnei isolates. Isolate susceptibility clustered at 8 mg/L, but three isolates displayed higher MICs (>64 mg/L) to azithromycin. All three isolates contained a plasmid-encoded mphA gene, known to encode a macrolide-2'-phosphotransferase enzyme. Transformation of the mphA gene into Escherichia coli DH10B allowed the transfer of decreased susceptibility to azithromycin. Although these isolates might traditionally be defined as resistant, there are no established breakpoints for resistance to confirm that treatment of these isolates with azithromycin would fail, which complicates susceptibility screening.