Two field experiments were carried out at the UAPNPBS experimental station, Seropedica, with two sorghum and one rice cultivars. The establishment, and inoculation effects, of Azospirillum spp. and Herbaspirillum strains marked with antibiotic resistance were investigated. One grain sorghum (BR 300) and one sugar sorghum (Br 505) cultivar were used. Azospirillum lipoferum strain S82 (isolated from surface sterilized roots of sorghum) established in both cultivars and comprised 40 to 80% of the Azospirillum spp. population in roots and stems 60 days after plant emergence (DAE). Azospirillum amazonense strain AmS91 (isolated from surface-sterilized roots of sorghum) reached only 50%. At 90 DAE, S82 almost disappeared (less than 30% of establishment) while the establishment of AmS91 remained constant in roots and stems. No establishment of H. Seropedicae strain H25 (isolated from surface-sterilized roots of sorghum) or A. lipoferum strain S65 (isolated from the root surface of sorghum) could be observed on inoculated roots. Inoculation with S82, AmS91 or S65 but not with H. Seropedicae H25, increased plant dry weight of both cultivars and total N in grain of the grain sorghum. In rice, A. lipoferum Al 121 and A. brasilense Sp 245 (isolated from surface sterilized rice and wheat roots respectively) established in the roots but there was no increase in Azospirillum spp. numbers due to inoculation. None of the strains affected plant growth or rice grain yield. Azospirillum amazonense A82 and H. Seropedicae Z95, which did not establish in roots, significantly enhanced seed germination.