Evidence for unequal lifetimes of the D/sup 0/ and D/sup +/

In the reaction e/sup +/e/sup -/..-->..psi''(3770), events containint either one or two electrons have been observed originating in semileptonic decays of D mesons. A comparison of these samples provides a determination of the branching ratios: b(D/sup 0/..-->..Xe..nu..) ..Xe..nu..)22.0 -2.2$(per-cent)=. These values imply that the ratio of D/sup 0/ and D/sup +/ lifetimes is tau(D/sup +/)/tau(D/sup 0/)>4.3 (at 95% confidence level).