Microwave rotational transitions for HMn(CO)5 with C-13 isotopic substitution in the axial and in one equatorial site were measured using a Flygare-Balle-type pulsed-beam spectrometer. Rotational constants obtained are A = B = 903.433(1) MHz for the axial (CO)-C-13 isotopomer and A = 902.278(3) MHz and B = 907.761(3) MHz for the isotopomer with one equatorial C-13 substitution. The new data are combined with previous data on HMn(CO)5 and DMn(CO)5 to obtain structural parameters for this complex. The Mn-H bond length obtained is r(Mn-H) = 1.65(2) angstrom and angleC(ax)-Mn-C(eq) = 97.0(5)-degrees.