Progress is reported on a project sponsored by the California Air Resources Board (ARB) to evaluate the potential of diesel particulate trap systems for retrofit application to reduce emissions from buses and trucks. 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. Field testing of this system on a transit coach is now ongoing at Southern California Rapid Transit District (SCRTD) and the experience is reported. Emission testing has been conducted regularly at ARB to verify the performance of the trap system. Work is also proceeding on the preparation of field testing of a similar system on a truck equipped with a Cummins' NTCC 400 diesel engine.