The effects of mixtures of rhizobium strains on the dry matter production of S. IOO white clover were investigated by growing plants in tubes of agar with four pure strains known to give contrasts in yield effectiveness, and comparing the results with those produced by plants grown with mixtures of these strains, and with those of uninoculated plants receiving mineral nitrogen. It was found that some components of the mixtures influenced yield much more than others and these strains therefore appeared to be strong competitors. Ability to infect and capacities to f u nitrogen were