Clinical dosimetry data are presented for the lower‐energy x‐ray beam of a Varian Clinac 1800 linear accelerator. This beam has comparable characteristics to single energy linear accelerators with the same stated 6‐MV x‐ray energy. The nominal beam energy was found to be 5.3±0.3 MV on the central axis. Beam quality expressed in terms of half‐value layer in water was found to vary by less than 10% over the entire field. The surface doses are only slightly, but consistently, larger than those reported in the literature for other 6‐MV linacs. Dosimetric results presented include central axis percentage depth dose (% DD) and tissue–maximum ratio (TMR), surface and buildup doses, output factors, and inverse square law applicability. The flatness and symmetry characteristics are within the manufacturer's specifications for both large and small fields.