Three experiments were performed on a fibre plant of the Malvaceae, Hibiscus cannabinus L., to determine growth responses to O3, the effect of development stage on sensitivity to O3 and interactions between effects of O3 and water stress. Plants were fumigated for 6 weeks at three concentrations of O3 in large hemispherical glasshouses at Lancaster, UK. H. cannabinus proved highly sensitive to O3 and showed both visible foliar injury and growth reductions which were statistically significant and correlated positively with O3 concentration (r>0.9 and P<0.001). Of the five different growth stages examined, i.e. 0-, 1-, 2-, 4- and 6-week-old stages, 4-week-old plants seemed to be most sensitive. Cotyledons were more susceptible to O3 injury than the true leaves, which in the younger stages were almost insensitive. O3 damage was alleviated by mild water stress but enhanced by severe water stress.