Observations of isoprene and its oxidation products methacrolein (MACR) and methyl vinyl ketone (MVK) are used to quantify the impact of isoprene oxidation on ozone production along the western slope of the Sierra Nevada mountains. Regular daytime up-slope wind flow patterns transport anthropogenic volatile organic compounds (VOC) and NOx emissions from the Central Valley toward the Sierra Nevada. A north--south band of oak forests stretching along the foothills and located approximately halfway between Sacramento and our measurement site (Blodgett Forest Research Station; elevation 1315 m) injects isoprene into this mixture. Subsequently, high ozone levels are encountered in these air masses. At Blodgett, daytime mixing ratios of isoprene's oxidation products and ozone were highly correlated. The observed daytime MVK/MACR ratio was used to estimate a mean of 9 (¿4) ¿ 106 molec. cm-3 between the measurement site and the Sierra foothills. The slope of the correlation between ozone and MVK was analyzed and compared to theoretical yield ratios for the photooxidation of isoprene to estimate the fraction of total ozone production due to isoprene oxidation. On average, over 40% of the observed midday ozone formation in this region was attributable to isoprene oxidation. On ozone episode days (maximum 3> > 90 ppb), the mean isoprene contribution was over 70%. The calculated isoprene contribution to ozone production was variable from day to day but tended to increase exponentially with both isoprene input and air temperature. NOx conditions in the up-slope air masses were very important in determining the ozone formation potential of isoprene, and the general dominance of isoprene as an ozone precursor suggests that summertime ozone abatement strategies for the region must focus on anthropogenic NOx rather than VOC reductions.