Nine native Frankia strains were isolated from root nodules of four chilean actinorhizal plants (Rhamnaceae). The strains were designated as ChI1, ChI2, ChI3 and ChI4 from Colletia hystrix; ReI4 and ReI6 from Retanilla ephedra; TqI12 and TqI15 from Talguenea quinquinervis and TtI42 from Trevoa trinervis. By scanning electron microscopy, all the strains exhibited similar actinomycetal structures: hyphae, sporangia and vesicles. The growth patterns of the isolates in BAP medium were similar. All showed a lag phase of approximately 6-7 days, then exhibited a logarithmic phase, except the ReI4 strain which seems to follow a linear growth pattern. A common feature of all the strains was a rapid loss of biomass at the end of the growth phase. All native strains grew on BAP medium supplemented with glucose. In six out of nine strains, the glucose was the best of the carbon sources tested. However, the strains differed in their ability to use other carbon sources such as arabinose, mannitol, maltose, succinate, sucrose, pyruvate, propionate and galactose. The isolates were sensitive to six antibiotics assayed (ampicillin, penicillin G, rifampicin, chloramphenicol, erythromycin and kanamycin). Using the acetylene reduction assay, the nitrogenase activity of the strains was determined. All strains grown in BAP medium lacking a combined nitrogen source were able to reduce acetylene 'in vitro'.