The frequency dependence of the ferromagnetic resonance (FMR) linewidth and effective linewidth has been measured in Mn doped single crystal barium hexaferrite (BaFe12−xMnxO19) withx=0.1. Mn doped samples were used to minimize conductivity losses. Measurements were made onc‐plane thin disks magnetized to saturation along the disk axis andcdirection. A shorted waveguide technique was used to measure the FMR profiles in the range of 55–90 GHz. The FMR field position increased linearly at 0.35 kOe/GHz, with a zero field extrapolation of 34.7 GHz as expected for uniform mode FMR in barium ferrite. The linewidth &Dgr;H increased linearly with frequency at 0.29 Oe/GHz with a zero frequency extrapolation of 18 Oe. A high‐Q cavity method was used to measure the high field effective linewidth, &Dgr;HeffFFat 10, 20, 35, and 60 GHz. The effective linewidth increased linearly with frequency at 0.4 Oe/GHz, with a zero frequency extrapolation of 4 Oe. There were no dependencies of &Dgr;H or &Dgr;HeffFFon sample thickness. These results indicate that (1) eddy current contributions to &Dgr;H and &Dgr;HeffFFin these Mn doped barium ferrite materials are absent, (2) &Dgr;HeffFFis significantly lower than &Dgr;H and appears to be a good measure of the intrinsic losses, and (3) both linewidths increase linearly with frequency at about the same rate.These intrinsic linewidths in barium ferrite are still significantly greater than linewidths in yttrium iron garnet. Research sponsored in part by the United States Office of Naval Research, Contract No. N00014‐90‐J‐4078. The single crystal samples were kindly provided by M. A. Wittenauer, Purdue University.