Two-dimensional (2-D) polyacrylamide gel electrophoresis was used to detect proteins induced in Frankia sp. strain ACN14a-tsr by root exudates of its symbiotic host, Alnus glutinosa. The 5 most prominent proteins were purified from 2-D gels and characterized by N-terminal sequencing. All of these proteins had a high percentage of similarity with known stress proteins. One protein match was the Fe superoxide dismutase (Fe-SOD), another was a tellurite resistance protein (Ter), the third was a bacterioferritin comigratory protein (Bcp); and two matches, differing only by their isoelectric point, were the same small heat shock protein (Hsp), a major immune reactive protein found in mycobacteria. This suggests that the symbiotic microorganism Frankia, first responds with a normal stress response to toxic root products of its symbiotic host plant. To confirm its identity, the gene corresponding to the Fe-SOD protein, sodF was isolated from a genomic library by a PCR-approach and sequenced. It is the first stress response gene characterized in Frankia.