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delta-aminolevulinic acid (ALA) is the obligatory precursor for tetrapyrroles and for chlorophylls in plants. Under illumination, these photosensitizers generate singlet oxygen, thus causing bleaching and death of treated tissues. We have examined whether superoxide is involved in the mode of action of ALA and whether SOD provides protection. Bean genotypes with similar carotenoid content but differing in SOD activity and cucumber seedlings were used throughout. Cucumber plants treated with 10 mM ethanolamine (EA) prior to ALA, had higher levels of chlorophyll fluorescence and lower values of electrolyte leakage than control. Bean cultivars with high SOD activity were considerably more tolerant to membrane damage caused by ALA than those with low SOD activity. SOD activity was greatly reduced in cucumber leaves treated with diethyldithiocarbamate (DDTC). Electrolyte leakage was markedly increased and chlorophyll fluorescence values were significantly lower in DDTC and ALA treated tissues as compared with those treated with ALA only. The results indicate that superoxide is involved in the toxicity caused by ALA and that, by breeding for high SOD activity, resistance to ALA can be achieved, thus allowing the use of ALA as a selective herbicide in the field.