Dietary change alters the ruminal ecosystem and can be regarded as a disturbance. Studying the response to a disturbance can help us understand the behavior of the ecosystem. Our work is concerned with the response of the ruminal ecosystem (composition and activities) during the application of repeated dietary disturbances to 6 dry Holstein cows. For 2 mo, the cows received a hay-based diet [experimental period (EP) 0], followed by 3 EP of successive changes (EP 1, 2, and 3) comprised of 2 parts: the first (10 d) with a corn silage-based diet and the second (25 d) with a hay-based diet. The measurements and samplings were done on the last days of EP 0 and of each part of EP 1 through 3, with the results of EP 0 used as covariables in the statistical models. The physicochemical measurements (pH and redox potential) and the fermentation variables (VFA, ammonia) were determined hourly between the morning and evening meals (n = 8 measurements/d). Samples of ruminal contents were taken 3 h after the morning meal to determine enzymatic activity [amylase, carboxymethyl cellulase (CMCase), and xylanase], to count the main protozoan genera and to quantify the bacteria by quantitative PCR, and to determine its structure by the capillary electrophoresis single-strand conformation polymorphism process. The pH fell for the corn silage-based diet with the EP (P < 0.05) but not for the hay-based diet. The VFA concentration decreased for both diets with the EP (P < 0.001), with the primary changes in acetate and propionate. The ammonia concentration increased for the corn silage-based diet with the EP (P < 0.05), whereas for the hay-based diet the highest value was observed for EP 2 (P < 0.05). The total quantity of bacteria decreased between EP 1 and 3 (P < 0.05) for both diets. The structure of the bacterial community was not affected by the disturbances for the corn silage-based diet, whereas for the hay-based diet large differences were evident between EP 1 and 3 (P < 0.05) and 2 and 3 (P < 0.01). The number of protozoa increased over the EP, with a more marked effect for the corn silage-based diet (diet x EP interaction, P < 0.05). The specific amylase, CMCase, and xylanase activities decreased over the EP for both diets (P < 0.05). The dietary changes applied in our experiment involved strong modifications of the ruminal ecosystem and alterations of ruminal fermentation and enzymatic activities. These alterations were reinforced with the repetition of the dietary changes.