Alnus rubra (Bong.) was grown for three months in a greenhouse and given 0, 6, 12, 31 and 62 migrograms cadmium/liter of nitrogen-free nutrient solution for 11 weeks. Segments of nodules and secondary roots were collected from plants grown at the different Cd levels. Thin sections were made for transmission electron microscopy and the ultrastructure of root and nodule cells was then investigated. Endophyte resorption advanced in the nodules ad the number of starch grains in root xylem parenchyma cells tripled as the cadmium concentrations increased from 12 to 62 micrograms/liter of nutrient solution. Nucleoli increased in prominence in the nuclei of the root xylem parenchyma cells as the cadmium concentration in the nutrient solution was raised from 12 to 62 micrograms/liter. Observations of root and nodule cell ultrastructure of Alnus rubra suggest that cadmium exerts its toxic effect in roots and nodules by decreasing the number of endophyte structures capable of fixing nitrogen and reducing the rate of respiration and growth by hose plant cells.