  An examination of leptons in ${\Upsilon (4S)}$ events tagged by reconstructed
$B$ decays yields semileptonic branching fractions of $b_-=(10.1 \pm 1.8\pm
1.4)\%$ for charged and $b_0=(10.9 \pm 0.7\pm 1.1)\%$ for neutral $B$ mesons.
This is the first measurement for charged $B$. Assuming equality of the charged
and neutral semileptonic widths, the ratio $b_-/b_0=0.93 \pm 0.18 \pm 0.12$ is
equivalent to the ratio of lifetimes. A postscript version is available through
World-Wide-Web in http://w4.lns.cornell.edu/public/CLNS/1994
