An objective of the 1997 Southern California Ozone Study (SCOS97) was to provide an up-to-date assessment of the importance of biogenic emissions for tropospheric ozone production in the South Coast Air Basin. To this end, ambient air samples were collected during September 1997 at the Azusa air-monitoring station for subsequent measurement of their radiocarbon (14C) content of the atmospheric nonmethane volatile organic compound (VOC) fraction. The 14C/12C ratio is proportional to the fraction of a sample's carbon that is biogenic. The proportionality constant was determined from local samples of vegetation, gasoline, and ambient CO2 collected during the same period. The median fraction of biogenic VOC observed from 0600 to 0900 hours (LT) was 7% (n = 5) with a range of -8% to 24%, from 1300 to 1600 hours it was 27% (n = 4) with a range of 11% to 39%, and from 1700 to 2000 hours it was 34 ¿ 7% for a single sample. On the basis of calculated 24-hour back trajectories the dominant source region for the air masses associated with periods of high biogenic VOC-C levels was a sector extending from the north to the east. Over all time and space that the samples represent, the median fraction of biogenic VOC was 18% (n = 10). Expressed as an atmospheric mixing ratio, the overall (median and 95% confidence interval) biogenic VOC-carbon contribution was 80 ¿ 50 nmol mol-1 which may be representative of the natural VOC-C background for the Los Angeles air basin.