The study was conducted at the ILRI-Debre Zeit Research Station to screen the variousrnextracts of Acacia anglls/issillla for evidence of toxicity using mixed and pure cultures ofrnbacteria, and to isolate bacteria capable of tolerating or detoxifYing the toxic principle. Gasrnproduction was llsed as an index of fermentation to screen the amino acid, non-basic,rnchloroform and ethanol extracts from A. anglls/issillla leaves for toxicity to mixed and purernrumen microbes. RlIlllinococclls a/blls 7, R. jlavefaciens FD-l, R. a/blls 1Il0Z a, Prevo/ellarnrlllllinico/a D31D, Se/enolllonas rlllllinantiulII D, Blltyrivibrio jlbriso/vens, and StreptococCllSrnbovis JB 1 were used as pure cultures. Alfalfa and elephant grass were used as substrates. Thernextracts were used at 0, 5, 10,20,30 and 50 % ofthe substrates. The samples were incubated atrn39Â°C for 120 h. Gas production, ammonia released and volatile fatty acid were estimated. Thernresult showed that ethanol, chloroform and non-basic extracts but not the amino acid were toxicrnto the mixed rumen microbes. All the three extracts inhibited the growth of all pure culturesrnexcept Se/enolllonas rlIl11inantlllll.rnExtracts containing tannin were more inhibitOlY to rumen bacteria suggesting that tanninrnwas one of the problem compound in A. anglls/issillla. Experiments were subsequently set tornisolate tannin tolerant or degrading bacteria. Tannin was extracted from A. angllstissillla andrnCalliandra ca/othyrslls leaves. Four straight rods and three spherical bacteria were isolatedrnusing roll tubes emiclled with tannic acid inoculated with rumen fluid from sheep and goats.rnThe isolates were characterized by morphology, products of fermentation and restrictionrnfragment length polymorphism (RFLP). Morphological, fermentative and RFLPrncharacterization indicate that the cocci isolates were Streptococclls while the rods were closely related to recently isolated tannin tolerant bacteria (ES 14.2) which had been shown to clusterrnwith Klebsiella