From september 1988 a fleet test started in Italy on urban buses equipped with a single ceramic monolith filter plus a by-pass and Diesel fuel burners for regeneration. The operation of the particulate trap, during the normal bus running in the city, has been assessed in three different ways: 1. Continuous control, by means of a data acquisition system purposely designed, of the main parameters affecting regeneration (i.e. time between consecutive regenerations, temperature in three selected zones in the filter during regeneration,filter loading); 2. Periodic control of smoke opacity of the bus measured, with and without the filter, under free acceleration and when bus is accelerating from 50 to 65 km/h on proving ground; 3. Particulate collection efficiency of the trap by using a dilution tunnel with the engine operated at the test bench. Information carried out from the fleet tests and additional information deriving from laboratory results were used for the system improvement.