Here we analyzed carboxymethylcellulase (CMCase: EC 3, 2, 1, 4), one of the key enzymes in the early symbiotic process, in Rhizobium. Specific immunogold labeling of electron microscopy was confirmed in Sinorhizobium fredii BCRC15769, ATCC35423, Sinorhizobium meliloti ATCC9930, and barely detected in Brodyrhizobium japonicum BCRC13528, ATCC10324 and Rhizobium rhizogenes ATCC11325. Non-specific labeling was detected in Rhizobium leguminosarum bv. viceae ATCC10004, Rhizobium leguminosarum bv. trifolii ATCC10328, and Mesorhizobium loti ATCC33669. Treatment of S. fredii BCRC15769 in the early log phase with the flavonoid genistein caused relocalization of CMCase. Together our data suggests a role for CMCase in early symbiosis. (c) 2008 Elsevier Ltd. All rights reserved.