Dock (Rumex obtusifolius) is commonly regarded as a weed, but it prevents bloat in cattle and the condensed tannins (CT) in dock are able to reduce the viability of gastrointestinal parasite eggs and larvae under in vitro conditions. These benefits, with other attributes of CT from a variety of forages promoted this study to examine effects of CT extracted from dock on the growth of five strains of proteolytic rumen bacteria in vitro. Streptococcus bovis NCFB 2476, Eubacterium sp. C124b, Prevotella bryantii B14, Butyrivibrio fibrisolvens H17c and Clostridium proteoclasticum B316^T were tested against 200, 400 and 600 µg CT/ml. In the absence of CT, all bacterial strains showed typical growth and reached maximum optical density (OD 600 nm) after 6 - 8 h of incubation at 39^oC. All strains continued to grow in the presence of 200 µg of the CT from dock per ml but attained significantly lower (P<0.001) OD 600 nm values than their counterparts in the control incubation. The addition of 400 and 600 µg CT/ml reduced (P<0.001) the growth of all bacterial strains tested compared to controls. All strains except P. bryantii did not initiate growth after the addition of 600 µg/ml relative to the growth before addition of CT. C. proteoclasticum was the most sensitive to the action of CT followed by B. fibrisolvens, S. bovis, Eubacterium sp. and P. bryantii. These results suggest that the dock CT have the ability to modify the growth of rumen proteolytic bacteria either directly or indirectly by preventing access to the protein.