Abstract : We report a measurement of the production cross section for b hadrons in p-pbar collisions at sqrt{s}=1.96 TeV. Using a data sample derived from an integrated luminosity of 83 pb^-1 collected with the upgraded Collider Detector (CDF II) at the Fermilab Tevatron, we analyze b hadrons, H_b, partially reconstructed in the semileptonic decay mode H_b -> mu^- D^0 X. Our measurement of the inclusive production cross section for b hadrons with transverse momentum p_T > 9 GeV/c and rapidity |y|<0.6 is sigma = 1.30 mub +/- 0.05 mub(stat) +/- 0.14 mub (syst) +/- 0.07 mub (B), where the uncertainties are statistical, systematic, and from branching fractions respectively. The differential cross sections dsigma/dp_T are found to be in good agreement with recent measurements of the H_b cross section and well described by fixed-order next-to-leading logarithm predictions.