From altogether 17 strains of Pseudomonas aeruginosa resistant to gentamicin, isolated in January 1975, 4 have been found to transfer their resistance to gentamicin and tobramycin, along with the resistance to other aminoglycosides, to rifampicin-resistant recipient strains of the same species. In some instances the transfer was strain-specific and therefore it was advisable to use more divergent recipient strains. It was found that a variant of a recipient strain (ML-4258) which already carries an R plasmid (found in the same area in 1972), accepted the Gm-R To-R plasmids with much lower frequencies than the original plasmid-free recipient. From this it may be concluded that R plasmids in P. aeruginosa from the Frankfurt area did not undergo significant genetic alterations within 3 years and thus they might be regarded as phylogenetically related.