Exceptionally high ozone ground level concentrations (>200 ppb) observed in Denver, Colorado, on the afternoon of March 4, 1978, are explained in terms of enhanced photochemistry combined with isentropic transport of ozone from the stratosphere. The maximum ozone concentrations observed on March 4 is the highest recorded for the 1975--1978 period. Inspection of the local pullutant and meterological data shows high early morning accumulation of precursor gases and enhanced potential for afternoon photochemical activity. Objective cross-sectional analysis and trajectories on isentropic surfaces indicate that some air parcels that mixed into the boundary layer over the city on the afternoon of March 4 were derived from a stratospheric intrusion 3 days earlier. It is concluded that a combination of these factors resulted in the exceptionally high ozone concentrations.