Heat-treated (100oC, 30 min) anaerobic sludge was used to convert cassava wastewater (10,000 mg COD/L) into hydrogen in an up-flow anaerobic sludge blanket (UASB) reactor. The operating hydraulic retention times (HRTs) were 24-4.8 h. A shift in HRT from 24 h to 12 h enhanced hydrogen production rate. When HRT was shortened from 12 h to 4.8 h, the percentage of hydrogen produced decreased from 46% to 10%. Peak of the hydrogen yield of 46 mL H2/g COD and the hydrogen production rate of 16.1 L/d were obtained at HRT 12 h. Granular sludge at HRT 12 h was a light-grey in color and was 0.14 mm in diameter. Each gram of biomass produced 0.89 mL H2/day. The endogenous decay coefficient (Kd) and yield coefficient (Yg) of granules were 0.64 /day and 0.93 g VSS/g COD, respectively. Responsible bacteria in granulated sludge were found to be diverse ranging from the phylum Firmicutes to Actinobacteria , Spirochaetes , and Bacteroidetes. Most of the sequences analyzed (11 out of 16) were classified as members of the Firmucutes. Among the phylum Firmucutes, most of the sequences (10 out of 11) felled into the Clostridia; Clostridiales. Change in DGGE band patterns implied that HRT affected directly the bacterial communities. From 5 samples with 5 different HRTs, bacteria communities can be classified into three patterns (24 -18 h, 12-8.4 h, and 4.8 h) indicating that there are HRT boundaries distinguishing bacterial communities in the reactors. At longer HRTs, more variety of bacteria was found.