Progress is reported on a project to evaluate the potential of diesel particulate trap systems for retrofit application to reduce emissions from buses and trucks in the State of California. A trap system was selected for evaluation, based on availability of commercial prototypes at the time of the project. Engine dynamometer testing of this trap system is reported over advanced design bus cycles. The results showed that the system is viable for application to a bus equipped with a Detroit Diesel Corporation 6V-92TA diesel engine, producing satisfactory exhaust backpressure and emission characteristics over a 500 hour test period. Work is now proceeding on field testing of this system on a transit coach, and engine dynamometer and field testing of a similar system on a truck equipped with a Cummins NTC 350 diesel engine.