Third-instar gypsy moth larvae were used to assess their feeding preference for white oak foliage which had been exposed to three concentrations of ozone. In separate experiments the insects preferred to feed on plant material exposed to the highest concentration of ozone (15 pphm). However, plant material exposed to the median concentration (9 pphm) was less preferred than the control (ambient air), which indicated a change in the chemistry of the foliage, making it less suitable as a host plant. A further experiment showed that this switch from preference to lack of preference occurred between 6 and 9 pphm ozone and reversed itself between 9 and 12 pphm.