We present observations of SN 1993J made with the Ryle Telescope at 15.25 GHz. These show radio emission becoming detectable on April 4, about 8d after the supernova explosion, and then rising at an approximately constant rate for the next 30d. The rate of increase then declines, until the flux density reaches a peak of ~ 80 mJy at about 75d after the explosion. Subsequently the emission shows a general decline, with variations on a time-scale of weeks. The general form of evolution is similar to that expected from `mini-shell' models of radio supernovae, although the long, steady rise in emission does not fit the models.