The standard model for radio supernovae considers the synchrotron radio emission to be only partially absorbed by ionized thermal electrons in the circumstellar wind of the progenitor star. However, for the best studied radio supernova, SN 1993J, we present evidence of synchrotron self-absorption based on modelfits to all available radio data. We show that while external absorption is the main absorption mechanism at early times, synchrotron self-absorption is the dominant mechanism at late epochs.