In this note oligopoly with iso-elastic demand is analysed. Unlike previous studies we consider general iso-elastic demand rather than the case of unit elasticity. An n-firm Nash-Cournot equilibrium for the case of heterogeneous constant marginal costs is derived. The main result is a closed-form solution that shows the dependency of the equilibrium on the elasticity of demand and the share of industry costs. The result has applications to a wide range of areas in oligopoly theory by allowing comparisons across markets with different elasticities of demand.