In order to determine the interactions between anaerobic bacteria and the performance of the upflow anaerobic sludge blanket, UASB, chemical oxygen demand, COD, volatile fatty acid, VFA, and methane gas productions were determined. The variations of anaerobic microorganisms in the anaerobic granular Sludge were counted under different operational conditions. The study was composed of two parts. In the first part, the numbers of methanogens and acedogens in the anaerobic granular sludge were counted at different COD removal efficiencies. The relationships between the numbers of methanogens, the methane gas production and VFA production were investigated. Most probable number (MPN) results showed that the mean number of total methanogens were between 15 x 10(4) and 29x 10(49) MPN/g at COD removal efficiencies of 33% and 82% in an UASB reactor treating 2,4 DCP at loading rates of 0.024-0.116 g/l.day, respectively. Decreases in methane gas percentage could be attributed to lowering in number of methanogens. The results showed that the COD removal efficiencies elevated from 33% to 82% while the number of total acedogens decreased from 120 cfu/ml to 8 cfu/ml as the 2,4 DCP loading rates lowered from 0.116 g/l.day to 0.024 g/l.day. There is a strong correlation between acedogen, methanogen bacteria numbers and reactor performances. In the second part, Methanobacterium bryanfii, Methanobacterium formicicum, Methanobrevibacter smithii, Methanococcus voltae, Methanosarcina mazei, Methanosarcina acetivorans, Methanogenium bourgense, and Methanospirillum hungatei were identified in the reactor treating 2,4 DCP.