  We observed SN1979C in M100 on 4 June 1999, about twenty years after
explosion, with a very sensitive four-antenna VLBI array at the wavelength of
18cm. The distance to M100 and the expansion velocities are such that the
supernova cannot be fully resolved by our Earth-wide array. Model-dependent
sizes for the source have been determined and compared with previous results.
We conclude that the supernova shock was initially in free expansion for 6 +/-
2 yrs and then experienced a very strong deceleration. The onset of
deceleration took place a few years before the abrupt trend change in the
integrated radio flux density curves. We estimate the shocked swept-up mass to
be about 1.6 solar masses, assuming a standard density profile for the CSM.
Such a swept-up mass for SN1979C suggests a mass of the hydrogen-rich envelope
ejected at explosion no larger than $M_{\rm env} \sim$ 0.9 solar masses. If
SN1979C originated in a binary star the low value of M_env suggests that the
companion of the progenitor star stripped off most of the hydrogen-rich
envelope mass of the presupernova star prior to the explosion.
