Photosynthesis of Phaseolus vulgaris L. leaves was measured after treatment with microcystin-LR (MC-LR), a potent cyanobacterial toxin and inhibitor of protein phosphatases 1 and 2A. The net photosynthetic rate fell by over 50% within 8 h of dipping leaves in MC-LR solution. This inhibitory effect was observed when leaves were treated once with concentrations above 10−2 mol m−3. At 10−2mol m−3, the inhibition caused by MC-LR was transient, and net photosynthesis rates essentially recovered after 5 d, whereas at 10−1 mol m−3, the net photosynthesis rate in treated leaves was still 42% of controls after 8 d. Necrosis was observed at the higher concentration, but not usually below 10−2 mol m−3. Analysis of net photosynthetic rate as a function of internal CO2 concentration and photosynthetically active photon flux density indicated that both the CO2-saturated rate of photosynthesis and the carboxylation efficiency were lowered when MC-LR-treated leaves were exposed to photosynthetically-saturating light. When the leaves were exposed to 2.0 × 10−5 mol m−3 MC-LR solution repeatedly, the photosynthetic rate was significantly reduced after 7 d, suggesting that intermittent exposure of P. vulgaris leaves to low concentrations of MC-LR brings about an effect on photosynthesis more inhibitory than that of a single exposure to high concentrations of MC-LR. These results indicate that relatively low concentrations of MC-LR cause damage to the photosynthetic apparatus of P. vulgaris in situ. We discuss the significance of these findings in relation to current spray irrigation practice for crop plants involving the use 6f water containing cyanobacterial blooms and microcystins.