Purpose: This work explores the effect of inoculation of the plant growth-promoting endophyte Streptomyces albidoflavus UYFA156 on the endophytic microbiota of its host, Festuca arundinaceae SFRO Don Tomás, and of another cultivar, Tacuabé, in which no growth promotion is observed. Methods: In vitro growth inhibition of fourteen bacterial endophytes from both cultivars was tested in dual cultures. Then, bacterial endophytic DNA was extracted from seeds and 7 and 30 days old plants of both cultivars with and without inoculation, with three replicates of each treatment. The V4 region of the 16S rRNA gene was amplified and sequenced through Ion Torrent and sequences were analyzed with R packages. Results: Diversity and composition in the seeds, along plant development, and in response to inoculation varied significantly. In 7 days old uninoculated plants, endophytic bacterial diversity was lower in cultivar SFRO Don Tomás than Tacuabé. However, upon inoculation, diversity in the cultivar SFRO Don Tomás increased significantly, approximately two-fold, with respect to uninoculated plants. On the other hand, no differences in diversity were observed in cultivar Tacuabé. Furthermore, microbiota composition varied significantly in plants of the cultivar SFRO Don Tomás upon inoculation, whereas this did not occur in Tacuabé plants. Conclusion: Growth promotion by the endophyte S. albidoflavus UYFA156 is accompanied by an early effect in bacterial microbiota diversity and composition associated with F. arundinaceae plants. These results encourage further explorations about the relationship between bacterial community modulation and plant growth promotion upon bacterial endophyte inoculation.