Experimental observations on the mode structure and beam divergence of a large‐bore high‐power argon ion laser are discussed. The gain medium, 185 cm in length and 12 mm in diameter, is capable of generating a steady‐state multiline multimode laser output of more than 125 W in the blue‐green. Various stable resonance cavity configurations have been investigated. Littrow prism and small aperture have also been used to obtain single‐line and low‐order transverse mode operations, respectively. The beam intensity profiles at two widely separated distances from the output coupling mirror are measured by scanning the beam across a small aperture affixed to a photodetector, using a rotating mirror. The two measured beam diameters are found to increase with the transverse mode volume (or output power) at a nearly constant ratio. The far‐field beam divergence angle deduced from these measurements is found to vary linearly with the initial beam diameter and to be of the order of 1 mrad at the highest output power.