  We report on VLBI observations of supernova 1986J in the spiral galaxy NGC
891 at two new epochs, 1990 July and 1999 February, t=7.4 and 15.9 yr after the
explosion, and on a comprehensive analysis of these and earlier observations
from t~4 yr after the explosion date, which we estimate to be 1983.2 +/- 1.1.
The source is a shell or composite, and continues to show a complex morphology
with large brightness modulations along the ridge and with protrusions. The
supernova is moderately to strongly decelerated. The average outer radius
expands as t^(0.71 +/- 0.11), and the expansion velocity has slowed to 6000
km/s at t=15.9 yr from an extrapolated 20,000 km/s at t=0.25 yr. The structure
changes significantly with time, showing that the evolution is not
self-similar. The shell structure is best visible at the latest epoch, when the
protrusions have diminished somewhat in prominence and a new, compact component
has appeared. The radio spectrum shows a clear inversion above 10 GHz. This
might be related to a pulsar nebula becoming visible through the debris of the
explosion. The radio flux density between 1.5 and 23 GHz decreases strongly
with time, with the flux density proportional to t^(-2.94 +/- 0.24) between
t~15 to 19 yr. This decrease is much more rapid than that found in earlier
measurements up to t~6 yr.
