In two experiments Festuca seeds (4 or 20 seeds/pot in first or second experiment respectively) were inoculated with Azospirillum strains (sp 7, mutant streptomycin resistant and G strain) in controlled laboratory conditions. Two illumination regimes were employed (197 and 274 microE. m-2.s-1, respectively). sp 7 cell number was determined during both experiments. The total number of diazotrophic bacteria was also determined on different dates. In both experiments the control pots received the same number of autoclaving bacteria. The results suggested that: (i) sp 7 number decreased in both experiments to 4 - 5% from initial number in 22 and 23 day after inoculation. This suggests that the plant density and illumination range employed did not affect the inoculum survival; (ii) the evolution of the number of different native diazotrophics strains, along the experiment, showed that this microbial number decrease in inoculated and control pots was related to the initial number.