The aim of this study was to investigate the impact of rumen content manipulation and its incubation in an in vitro system on the abundance of some microbial groups and the bacterial diversity of goat rumens. Animals and single-flow continuous-culture fermenters were fed diets differing in forage to concentrate ratio (70:30; LC and 30:70; HC). Rumen contents were sampled after animals' adaptation to the experimental diets, processed for inoculum preparation and inoculated into fermenters. Fermenter contents were sampled 1 and 7 days after inoculation. Total bacteria, Fibrobacter succinogenes, fungi and methanogen abundances were lower in the fermenter than in goat rumens, but no differences were found for Ruminococcus flavefaciens. The abundances of all these microorganisms were similar at 1 and 7 days of rumen content incubation in fermenters. Bacterial species richness did not change due to rumen content processing or the in vitro incubation. ShannonWiener index and Pielou evenness were lower in the fermenter than in rumen only when the enzyme HaeIII was used in terminal-restriction fragment length polymorphism analysis. Non-metric multidimensional scaling analysis, both in denaturing gradient gel electrophoresis and terminal-restriction fragment length polymorphism, showed a segregation of in vivo and in vitro samples, but no trends of grouping for fermenter samples was observed. The HC diet promoted higher abundance of total bacteria than LC in rumen but not in fermenters. Diet only had an effect on bacterial diversity when the enzyme HaeIII was considered. Rumen content processing and incubation in fermenters caused an important decline of the studied ruminal microbial groups although bacterial community structure and diversity did not significantly change.