Spruce shoot aphid, Cinara pilicornis Hartig (Homoptera: Lachnidae), is an aphid species that has shown enhanced performance on trees exposed to SO2 or mixtures of air pollutants, whereas results with ozone have been contradictory. Using a 4-week chamber fumigation experiment, we tested how different population dynamic parameters of aphids are affected by ozone (O3). Mean relative growth rate (MRGR), development time and reproduction rate of C. pilicornis were determined using 7h day-time O3 concentrations of 0, 40, 80 and 160 ppb. Development period from birth to reproductive stage was fastest at 80 ppb during early shoot elongation, while reproduction and intrinsic rate of population increase was not significantly affected by ozone concentration. There was a significant negative correlation between ozone concentration and the MRGR of first instar nymphs in the third fumigation week. In feeding test performed on seedlings after fumigation, the MRGR was reduced on shoots which had been exposed to 80 ppb O3. The results suggest that elevated O3 concentration during early shoot elongation period may stimulate population development of C. pilicornis, but on maturing shoots, high O3 concentration has a negative effect on aphid performance. This might be due to accelerated ageing of O3 exposed shoots.