Cross-inoculation experiments with 10 pure cultured strains and 17 host species were carried out. The 10 strains were isolated from the root nodules on actinorhizal trees ranging in 9 species, 5 genera and 4 families. The host species belong to 5 genera. The pure cultured strains from Alnus are of strong ability to infect different species of the same genus. The seedlings inoculated with these strains are able to nodulate normally. These strains can also infect and nodulate the seedlings of Myrica californica, but not the seedlings of Elaeagnus, Casuarina and Myrica rubra. The pure cultured strains from Elaeagnus can infect and nodulate the host species in the same genus and family with an exception of E. viridis var. delavayi, which can be only poorly nodulated by a few strains from Elaeagnus. The strains from Elaeagnus cannot infect the seedlings of Alnus and Myrica rubra. The results presented here suggest that Frankia endophytes can be divided into two groups: Alnus group and Elaeagnus group.