Colonization of sorghum and wheat after seed inoculation with Gluconacetobacter diazotrophicus strains PAL 5 and UAP 5541/pRGS561 (containing the marker gene gusA) was studied by colony counting and microscopic observation of plant tissues. Inoculum levels as low as 10² CFU per seed were enough for root colonization and further spreading in aerial tissues. Rhizoplane colonization was around 7 log CFU g^-1 (fresh weight). G. diazotrophicus was found inside sorghum and wheat roots with populations higher than 5 log CFU g^-1 (fresh weight). Stem colonization remained stable for 30 days post inoculation with endophyte concentrations from 4 to 5 log CFU g^-1 (fresh weight) (in both plants). Population in leaves decreased continuously being undetectable after 17 days post inoculation.