We have previously reported detection of significant pyrroloquinoline quinone-linked glucose dehydrogenase activity in Sinorhizobium meliloti cells isolated from alfalfa (Medicago sativa L.) nodules. In this work, we report the expression of the gcd gene (SMc00110) during root nodule development and characterize the symbiotic phenotype of S. meliloti gcd mutant RmH580. Using a S. meliloti strain carrying a gcd–lacZ transcriptional fusion, gcd expression was detected from very early stages of plant–bacteria interactions, at the rhizosphere level, and during further stages of nodule development. Alfalfa plants inoculated with RmH580 showed a delay in nodule emergence and a reduced ability for nodulation at various inoculum dosages. RmH580 was also deficient in its competitive ability; in coinoculation experiments a mutant:wild-type inoculum ratio higher than 100:1 was necessary to obtain an equal ratio of nodule occupancy. These results indicate that PQQ-linked glucose dehydrogenase is required by S. meliloti for optimal nodulation efficiency and competitiveness on alfalfa roots.