Seven of eight Rhizobium strains survived 70° C but not 80° C for half an hour in liquid media. The proportion of cells surviving heat treatment varied with the composition of the media and the age of the cultures, but did never exceed 0.7%. It appears that this heat resistance is not an effect of sporulation, according to the classical definition of a spore. Spore staining gave always negative results. However, spore-like structures could be seen in cells of heat resistant cultures. But it was not possible to correlate the number of these cells with the number of heat resistant cells. There was no difference between cells surviving heat treatment and untreated cells concerning infectivity and effectivity.