  We present a measurement of the B_c+ meson lifetime in the semileptonic decay
mode Bc+ ->J/Psi e+ nu using the CDF II detector at the Fermilab Tevatron
Collider. From a sample of about 360pb-1 p-pbar collisions at sqrt(s)=1.96TeV,
we reconstruct J/Psi e+ pairs with invariant mass in the kinematically allowed
range 4<M(J/Psi e)<6GeV/c^2. A fit to the decay-length distribution of 238
signal events yields a measured Bc mesion lifetime 0.463 (+0.073,-0.065)+-0.036
ps.
