An adhesion‐defective mutant of Ruminococcus albus SY3 was isolated by a subtractive enrichment procedure, which involved repetitive adsorption of cellobiose‐grown cells to cellulose. The growth characteristics of the mutant were compared with those of the wild type. Like the wild‐type cells, the mutant was capable of growing on soluble substrates, i.e. cellobiose and xylan. However, in contrast to the wild type strain, the mutant was impaired in its capacity to utilize insoluble substrates, e.g. crystalline cellulose, acid‐swollen cellulose or alfalfa cell walls. Scanning electron microscopy revealed protuberance‐like surface structures on the wild‐type strain which were absent on the mutant. The levels of endoglucanase and xylanase enzymatic activities released into the extracellular culture fluid were higher in the wild type compared to the mutant. However, Avicelase activity was not detected in the extracellular culture fluid of either strains when grown on cellobiose.