Public awareness of center city bus emissions is high, especially on transit ways where at peak load times a bus passes every 15 seconds. To investigate the potential reduction in smoke and odor levels, a typical bus with a 6V71 engine was equipped with a Catalytic Trap Oxidizer. An in-service test procedure was developed and the reductions in measured smoke and subjective odor were significant. The bus was returned to revenue service in Philadelphia at an average speed of 7.5 mph. The effects of the CTO on smoke, smell, fuel consumption and maintenance during service are reported.