Concentrations of exchangeable and easily reducible manganese (Mn) were measured monthly for 3 1 months in acidic soils at 6 sites used for cropping in north-eastern Victoria. Manganese concentrations in shoots of wheat (Triticum aestivum), when present, were also measured. Changes in Mn concentrations in the soils and in wheat plants were related to seasonal conditions and to the occurrence of crop yellowing, a chlorotic and stunting disorder which affects cereals grown in the region. Peaks in the concentration of exchangeable Mn occurred either when the soils were waterlogged for a prolonged period or when they were subjected to extreme heating and drying during summer. Shoot concentrations of up to 1200 mg/kg of Mn were measured for plants growing in waterlogged soils. However, in some cases concentrations less than 400 mg/kg, a concentration not considered harmful, were measured in shoots of wheat growing under waterlogged conditions but which still showed symptoms of severe chlorosis and stunting. In the absence of waterlogging, concentrations of Mn in wheat shoots were usually less than 400 mg/kg. The evidence suggests that Mn toxicity is not the primary cause of crop yellowing in north-eastern Victoria, but Mn toxicity, induced by waterlogging, probably contributes to crop yellowing when waterlogging is severe. Soil acidity did not appear to be high enough to cause Mn toxicity in the absence of waterlogging.