It is shown that a Josephson junction, biased at a voltageV0and driven simultaneously by two monochromatic sources at the frequencies &ohgr;1and &ohgr;2, produces an output power at the frequency &ohgr;3= &ohgr;1—n&ohgr;2and that this frequency is independent of the Josephson frequency &ohgr;0= 2eV0/ℏ. The power output depends upon the dc bias, showing fine structure with a voltage periodicity of ℏ&ohgr;2/2e.