The sensitivity of 12 Frankia strains to heavy metals was determined by a growth inhibition assay. In general, all of the strains were sensitive to low concentrations (<0.5 mM) of Ag1+, AsO2
1−, Cd2+, SbO2
1−, and Ni2+, but most of the strains were less sensitive to Pb2+ (6 to 8 mM), CrO4
2− (1.0 to 1.75 mM), AsO4
3− (>50 mM), and SeO2
2− (1.5 to 3.5 mM). While most strains were sensitive to 0.1 mM Cu2+, four strains were resistant to elevated levels of Cu2+ (2 to 5 mM and concentrations as high as 20 mM). The mechanism of SeO2
2− resistance seems to involve reduction of the selenite oxyanion to insoluble elemental selenium, whereas Pb2+ resistance and Cu2+ resistance may involve sequestration or binding mechanisms. Indications of the resistance mechanisms for the other heavy metals were not as clear.