A low wattage mercury vapor lamp comprises a discharge vessel (10) including a tubular body (12), first and second electrodes (20, 22) and two end walls (16, 18), which close opposite ends of the tubular body. The discharge vessel contains an ionizable fill material having a concentration of mercury of from 0.10 to 0.20 mg/mm.sup.3. The discharge vessel operates at elevated pressures of from 80 to 170 atmospheres at power of 20 watts resulting in improved lumen maintenance and reduced through-life color shift.