During a visit to a farm on December 15, 1980, in the Morro Agudo County of the State of São Paulo, yellow to yellowish patches were observed in about 20 hectares of soybean planted with the cultivar Bossier. Plants that showed symptoms of toxicity were together with plants without symptoms. The symptoms initiated from the third trifoliate leaves upwards. These symptoms were also observed in the neighbouring farms that planted Bossier cultivar. After a period of approximately 20 days the symptoms disappeared. Leaf analysis showed high concentration of Fe in the leaves with symptoms whereas without symptoms the iron concentration was satisfactory. A possible explanation of this toxicity is due to the fact that it rained 439 mm in the month of November when the normal rainfall is 170 mm. With the excess of rainfall iron (III) coumpounds could be reduced to Fe2+ ion which is preferentially absorbed by the plant.