We have investigated the influence of biogenic emissions on ozone (O3) formation in Houston, Texas using a chemical transport model. Isoprene emissions play an important role in O3 formation when the O3 plume occurs in the afternoon in the urban Houston area. The O3 concentration is reduced by 20-40 ppb when the isoprene emission is removed in the CTM simulations. However, the decrease in the O3 concentration over the industrial Ship Channel area due to exclusion of isoprene emissions is less than 10 ppb. Additional sensitivity studies show that the surface O3 decrease results primarily from local isoprene emissions, although transport of isoprene from the north of the urban Houston area is found to be non-negligible in the isoprene budget on several days. The contribution of monoterpene emissions to O3 formation is insignificant due to its low emission rate and relatively slow reaction rate.