  The radio source M81* at the core of the nearby spiral galaxy M81 is a
low-luminosity active galactic nucleus. The close distance of 3.63Mpc allows
its morphology to be studied in great detail. Here we present preliminary
results from continuum 7 mm VLBI observations of its core, using
phase-referencing techniques. These observations set constrains on the size of
M81* at this frequency and enable us to test the frequency dependence on its
physical properties.
