Little is known regarding why some strains of rhizobia fail to form successful symbiotic associations with leguminous crop plants. Melino et al. compare incompatible (no N2-fixation) and poorly compatible (reduced N2-fixation) associations between four genotypes of clover (Trifolium spp.) and four strains of Rhizobium leguminosarum bv. trifolii and assess symbiotic effectiveness by a variety of measures. They find three major responses to incompatibility, namely failed bacterial endocytosis from infection threads into plant cortical cells, bacteroid differentiation that aborts prematurely, and a reduced pool of functional bacteroids as a result of premature senescence. Thus poor plant growth responses resulting from nodulation by incompatible strains can now be linked to specific morphological aberrations.