Nine different growth media were evaluated to determine the best growth conditions to achieve cultures of a high cell number of fast-growing rhizobia to produce inoculants. We found that <i>Sinorhizobium fredii</i> strains have complex nutritional requirements that were fulfilled by adding to the media 4 g <i>Amaranthus cruentus</i> L. seed meal/l. The survival of fast growing strains is a variable trait, but those strains that survived at high levels even after 6-month storage, hypernodulated soybeans and fixed atmospheric nitrogen at levels as high as those of <i>Bradyrhizobium japonicum</i>.Instituto de Fisiología Vegetal