A medium size diesel engine converted to natural gas operation on the Otto principle has been studied under stoichiometric and lean burn operation in order to evaluate the potential to reduce the NOX exhaust gas emissions below the stringent limit prescribed by the Swiss Federal Clean Air Act - 250 mg/mN3, 5 % O2 (at normal (N) conditions, 5 % residual oxygen and dry). While both operational modes fulfill the prescribed NOX limit, lean burn operation, combined with turbocharging, provides a higher brake power and a better fuel conversion efficiency.