Enterobacter cloacae GS1 was isolated by in-planta enrichment of a rice rhizoplane bacterial community. It displayed strong seed adherence ability (2.5 × 10 5 cfu/seed) and colonized rice roots reaching up to 1.65 × 10 9 cfu/g of fresh root weight in a gnotobiotic root colonization system. E. cloacae GS1 was motile, able to solubilize tricalcium phosphate, and produced indole acetic acid like substances (15 μg/ml). As an introduced bioinoculant in non-sterile soil, E. cloacae GS1 colonized rice roots and significantly improved the fresh weight, root length, shoot length, and nitrogen content in inoculated rice seedlings as compared to uninoculated controls. This isolate was tagged with green fluorescent protein and various stages of root colonization in gnotobiotic hydroponic environment and non-sterile soil environment were followed by fluorescence microscopy. Owing to its effective root colonizing ability and growth promoting potential, Enterobacter cloacae GS1 is a promising symbiotic bioinoculant for rice. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.