Ozone concentrations in Britain vary greatly from year to year. In the mid-1980s they were relatively low but in 1989 and 1990 the number of hours when concentrations exceeded 60 nl 1−1 was 3–4 times greater than in the preceding 2 yr. Previous experiments suggested that this might have resulted in an increase in ozone resistance of Plant ago major L. populations, so a comparison was made of seed-grown plants collected from the same three sites in Derbyshire/South Yorkshire in the 1980s and 1991. Ozone resistance was measured by exposure to charcoal-filtered air or air containing 70 nl 1−O3 (7 h d−1) and expressed as R%, the mean relative growth rate in ozone expressed as a% of that in charcoal-filtered air. Ozone resistance changed significantly in two of the populations (ISP and Totley) between the years 1985 and 1991. There was no significant change in the resistance of the Scaftworth population between 1988 and 1991 when expressed as R% but there were significant physiological differences between the collections. The changes in R% were accompanied by differences in the response of the stomata to ozone and in effects on dark respiration, but not in assimilatory capacity. This is the first report of a change in ozone resistance in populations over a short period of time. It is not known if the 1 991 plants were the descendants of the 1985/88 plants or whether there had been an invasion by new genotypes from outside the area. The role of other environmental factors in determining ozone resistance, and the implications of the data for the definition and mapping of critical levels are both discussed. It is predicted that if the reported changes were caused by ozone, the region where future changes in ozone resistance are most likely to occur is that between the Midlands and the north of England.