  We report the preparation method of, and boron isotope effect for MgB$_2$, a
new binary intermetallic superconductor with a remarkably high superconducting
transition temperature $T_c$($^{10}$B) = 40.2 K. Measurements of both
temperature dependent magnetization and specific heat reveal a 1.0 K shift in
$T_c$ between Mg$^{11}$B$_2$ and Mg$^{10}$B$_2$. Whereas such a high transition
temperature might imply exotic coupling mechanisms, the boron isotope effect in
MgB$_2$ is consistent with the material being a phonon-mediated BCS
superconductor.
