Frankia from root nodules of nine different species of Ceanothus were characterized. DNA was amplified directly from nodular material using the polymerase chain reaction (PCR). The amplified region includes the 3' end of the 16S rRNA gene, the intergenic spacer (IGS), and a large portion of the 23S rRNA gene. Restriction enzyme digestions of PCR products allowed us to designate PCR-RFLP groups among the Ceanothus-infective Frankia tested. The groupings did not follow the taxonomic lines of the Ceanothus host species. Instead, the Frankia strains present followed a geographical pattern. This information was used to choose representative Ceanothus-infective Frankia for phylogenetic analysis. Full-length 16S rDNA sequences were amplified directly from the nodules of two Ceanothus species using the PCR. Sequences were determined using an automated sequencer, compared against GenBank, and assembled into consensus sequences. The sequences were aligned with other full-length Frankia 16S rDNA sequences available from the database. Phylogenetic trees were obtained from three different algorithms: neighbor joining, parsimony, and the maximum-likelihood method. These Ceanothus microsymbionts appear to be most closely related to the microsymbiont associated with Dryas drummondii using all three methods. / Graduation date: 1998