We report the characterization of an intrinsic, chromosomally carried aph(3')-IIc gene from Stenotrophomonas maltophilia clinical isolate K279a, encoding an aminoglycoside phosphotransferase enzyme that significantly increases MICs of kanamycin, neomycin, butirosin, and paromomycin when expressed in Escherichia coli. Disruption of aph(3')-IIc in K279a results in decreased MICs of these drugs.