Strains of Rhizobium leguminosarum (biovar trifolil) isolated from two Ethiopian soils or obtained from a
commercial source were evaluated for symbiotic effectiveness on five African annual clover species. Numerous
Rhizobium trifolii strains that exhibited varying levels of symbiotic effectiveness were isolated from both soils
(a nitosol and a vertisol), and it was possible to identify strains that were highly effective for each clover species.
The soil isolates were, as a group, superior to the strains from the commercial source. Several R. trifolii strains
were found to be effective on more than one clover species, and there appeared to be at least two and possibly
three distinct cross-inoculation effectiveness groups.