The effect of replacing concentrate with 2 different feed blocks (FB) on ruminal fermentation and microbial growth was evaluated in goats and in single-flow continuous-culture fermenters. Diets consisted of alfalfa hay plus concentrate and alfalfa hay plus concentrate with 1 of the 2 studied FB. Three trials were carried out with 6 rumen-fistulated Granadina goats and 3 incubation runs in 6 single-flow continuous-culture fermenters. Experimental treatments were assigned randomly within each run, with 2 repetitions for each diet. At the end of each in vivo trial, the rumen contents were obtained for inoculating the fermenters. For each incubation run, the fermenters were inoculated with ruminal fluid from goats fed the same diet supplied to the corresponding fermenter flask. The average pH values, total and individual VFA, and NH3-N concentrations, and acetate: propionate ratios in the rumen of goats were not affected (P >= 0.10) by diet, whereas the microbial N flow (MNF) and efficiency were affected (P <= 0.001), with the greatest values observed for the diet without FB. In fermenters, the diet affected pH (P < 0.001), propionate concentrations (P = 0.01), acetate: propionate ratio (P = 0.03), carbohydrate digestibility (P = 0.05), and total (P = 0.02), NH3 (P = 0.005), and non-NH3 (P = 0.02) N flows, whereas the efficiency of VFA production was not affected (P = 0.75). The effect of diet on MNF and efficiency depended on the bacterial pellet used as a reference. An effect (P < 0.05) of diet on the composition of solid- and liquid-associated bacteria was observed. The compositions of liquid-associated bacteria in the fermenter contents and effluent were similar (P = 0.05). Differences (P < 0.001) between in vivo and in vitro values for most fermentation variables and bacterial pellet compositions were found. Partial replacement of the concentrate with FB did not greatly compromise carbohydrate fermentation in unproductive goats. However, this was not the case for MNF and efficiency. Differences between the results obtained in vivo and in vitro indicate a need to identify conditions in fermenters that allow better simulation of fermentation, microbial growth, and bacterial pellet composition in vivo. Reduced feeding cost could be achieved with the inclusion of FB in the diets of unproductive goats without altering rumen fermentation.