  We report on a measurement of the highly forbidden $6s^2 ^1S_0 \to 5d6s ^3
D_1$ magnetic-dipole transition in atomic ytterbium using the
Stark-interference technique. This amplitude is important in interpreting a
future parity nonconservation experiment that exploits the same transition. We
find $| < 5d6s ^3 D_1 | M1 | 6s^2 ^1S_0 > | ~ = ~ 1.33(6)_{Stat}(20)_{\beta}
\times 10^{-4} \mu_0$, where the larger uncertainty comes from the previously
measured vector transition polarizability $\beta$. The $M1$ amplitude is small
and should not limit the precision of the parity nonconservation experiment.
