Phenotypic diversity of rhizobia has been studied by several methods, particularly numerical analysis, enzyme pattern and serological study. Numerical analysis employs a large range of biochemical and metabolic tests to differentiate among rhizobial species. The results obtained from numerical analysis support the proposal of several novel species of rhizobia. Rhizobial species vary in their enzymatic production and several enzymes are found to be necessary for the symbiotic effectiveness. Serological techniques are the most specific methods for identifying rhizobia based on natural marker characteristics. Phenotypic characterization by serotyping has been used to explore rhizobial populations in different geograpical origins. Rhizobia in different serogroups have been found predominant among field populations.