  20 months of observations of the radio emission at 15 GHz from Cygnus X-1,
starting in 1996 October, show variations at the binary period of 5.6 days, but
with a phase offset from those at X-ray wavelengths. There are also longer-term
variations on a time-scale of 150 days which are only loosely related to the
soft X-ray flux. The source was in the hard/low X-ray state throughout this
period. The mean 15-GHz flux density is 13 mJy, the radio spectrum is flat, and
the semi-amplitude of the orbital modulation about 2 mJy. We discuss the
possible origins of the modulation and the relationship to the soft X-ray
emission.
