There have been few studies, to date, examining the effect of seed sludge on the microbial community established in a new anaerobic digestion (AD) system and whether or not the population present in the seed sludge establishes it self as the predominant population. Further, no reported studies have yet examined the differences in microbial populations that result from the formation of granular biomass in upflow anaerobic sludge blanket (UASB) systems. This study focused on examining the changes in microbial diversity between the initial seed sludge and the community that becomes established in a new digester. Using 16S rRNA clone libraries the diversity of microbes in both the granular and liquid biomass fractions from 3 AD sludge samples was examined and compared. Results showed that each sample had unique microbial community, with the distribution of sequences at the phylum level highly variable. This suggests that the feedstock had an effect of enriching microbial populations that are uniquely suited to a particular feedstock. Differences between the granular and liquid biomass fractions of each sample were less pronounced than differences attributable to the change in feedstock, however the results suggest that there are different functional groups in each fraction. (C) 2011 Elsevier Ltd. All rights reserved.