SUMMARYAn investigation concerning double strain occupancy of nodules and the competitive ability for nodulation ofRhizobium trifoliistrains 7Astrrand 32spcrwas carried out using red and white clover varieties grown in agar and soil.Only two nodules of the 498 typed from plants grown in soil contained both inoculated strains and none of those analysed by the replica plating technique were formed by both 32spcrand an indigenous strain. In contrast, plants grown on agar produced between 17% and 25% doubly infected nodules. Strain 32spcrwas more competitive for nodulation than 7Astrrboth in soil and agar. Indigenous rhizobia in soil were highly competitive for nodulation with the introduced strains; increasing the density of the inoculum resulted in an increase in the proportion of nodules occupied by the inoculated strains. The strains used as mixed inocula in soil were less successful in nodulation than the same strains used as pure inocula. The results also indicate that there was a host effect on the nodulation success of 7Astrr.