Proportions of cellulolytic and amylolytic bacteria in particle associated rumen bacteria were estimated from odd and branched chain fatty acid patterns. Whole rumen contents were collected from four dairy cows and particle associated bacteria were separated by differential centrifugation. The experiment was according to a 4×4 Latin square design with diets differing in forage:concentrate ratio, ranging from 80:20 to 35:65. Proportions of cellulolytic bacteria decreased with increasing dietary proportion of concentrate and estimates were in general agreement with population assessments obtained both by traditional bacterial enumerations and more recent molecular techniques.