We present a measurement of the B(s)0 lifetime in
fully and partially reconstructed B(s)0→Ds-(ϕπ-)X decays in
1.3 fb-1 collected in pp̅ collisions at √s=1.96 TeV
by the CDF II detector at the Fermilab Tevatron. We measure
τ(B(s)0)=1.518±0.041(stat)±0.027(syst) ps. The
ratio of this result and the world average B0 lifetime yields
τ(B(s)0)/τ(B0)=0.99±0.03, which is in agreement with recent theoretical
predictions.