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Field-grown beati crops {Phaseolus vulgaris L.) were exposed to either ambient non-filtered or filtered air in opentop chambers at two sites in the Po plain (Northern Italy) in 1988 and 1989. Three cultivars were utilized in 1988 (Taylor's Horticultural, Lingua di Fuoco, Saluggia), one in 1989 (Taylor's Horticultural). The crops grown in ambient non-filtered air showed significant reductions in yield compared with those in filtered air at both sites for the two years. Yield decrement of the most sensitive cultivar (Taylor's Horticultural) reached 31 "o at the urban site in 1988 and 31 °o at the rural site in 1989. Severe foliar injury attributable to oxidants was observed on plants kept in non-filtered air or grown in non-chambered control plots. The instrumental monitoring of air pollutants showed 7 h means of O3 up to 50 nl T^ during the experiments; it is known that these values can cause yield losses to sensitive crops. The concentrations of other pollutants (SO^ and NO,) were on the contrary below the known thresholds for yield losses to plants. We can argue that ambient ozone is the major air contaminant responsible for the observed yield losses of bean; the negative effects can occur both at rural and urban sites because of the widespread presence of photooxidants in the Po plain.