During 1994-95 and 1995-96, genetically modified (GM) strains of Rhizobium leguminosarumbv. viciae and Azospirillum brasilense along with parental non-modified wild types were field-tested as biofertilizers and phytostimulators, respectively. Their fate was assessed, in terms of environmental impact, with respect to soil/rhizosphere microbiota and biochemical soil/plant activities. Determinations included soil microbial biomass, soil metabolic activities (respiration, N₂O emission), bacterial populations in the rhizosphere and soil, fluorescent pseudomonads, vesicular-arbuscular mycorrhizae. The experimental design of field tests allowed to compare the effects of bacterial strains with high and low colonizing ability along with strains containing the reporter genes as stable and unstable constructs. The performance consistency of the inoculants was also evaluated by means of statistical analysis across multiple field trials, including the assessment of effects on crops.