Abstract The effect of diesel vehicle elemental carbon on visibility in California is estimated for 1980 and projected to the early 1990's. An emissions budget model indicates that heavy-duty diesel trucks contributed nearly one-half of statewide elemental carbon emissions in 1980 and about 5–15% of statewide light extinction (visibility reduction). A lead tracer model indicates somewhat larger extinction contributions from heavy-duty diesel trucks, about 5 to 25% statewide in 1980. Even greater visibility impacts (too large to be reasonable) are suggested by a CO tracer model. Because of increased diesel usage — due to both overall traffic growth and partial conversion of the vehicle fleet to diesels (10% of light-duty, 20% of medium-duty, and 60% of currently gasoline heavy-duty) — visibility in California is projected to decrease significantly (about 9 to 35%) from 1980 to the early 1990's under a “no control” scenario. Diesel vehicle elemental carbon would then contribute 13 to 40% of statewide visibility reduction. Even in the early 1990's, heavy-duty trucks would account for about three-fourths of the emissions from the entire diesel fleet. The most uncertain aspect of the conclusions is the overall magnitude of the visibility effect predicted by the emission budget and Pb tracer models. The partition of visibility impacts among light-, medium-, and heavy-duty vehicles is more definite. The conclusions are rather insensitive to reasonable changes in the assumptions regarding emission factors, traffic growth, and dieselization percentages. The findings indicate that, in order to be effective in protecting visibility, the current California standards for light- and medium-duty vehicles would have to be extended to include regulations for heavy-duty trucks.