A data base was obtained for comparing the benzo(a)pyrene (BaP) emission rates from diesel automobiles with those from current-production, catalyst and noncatalyst gasoline automobiles. Exhaust particulate samples were collected on filters, using a large emission tunnel. The extractable portion of the exhaust particulate was removed by Soxhlet extraction, with a mixture of benzene and ethanol. The BaP determination was made by high-performance liquid chromatography, using fluorescence detection. The BaP emission rate from an Oldsmobile diesel car was the same as the average rate from seven noncatalyst gasoline cars, 2.7 micrograms per mile. The BaP mass concentration in the diesel particulate was five parts per million, about twenty times lower than that in noncatalyst gasoline particulate. BaP emission rates showed no simple correlation with hydrocarbon or carbon monoxide emission rates. The catalytic converter reduced BaP emission by more than 95% on gasoline cars, but had no effect on BaP emission from a diesel car.