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Notes: Abstract The response of a glucose-fed anaerobic chemostat community to a long-term (〉200 days) periodic substrate perturbation was examined. Identical steady-state communities were established in a ``mother'' and a ``daughter'' reactor at pH 7, 35°C, inlet glucose concentration of 8 g l−1, and dilution rate of 0.1 day−1. After reaching steady state, the daughter reactor was subjected to a periodic organic loading pattern in which the influent glucose concentration was alternately varied from 16 g l−1 to 0 g l−1 (mineral media only) on a 2-day cycle (1 day at 16 g l−1 followed by 1 day without glucose feed). The average organic loading rate for the perturbation cycle was equal to the steady-state glucose loading rate of 0.8 g l−1 day−1. The dilution rate of the daughter reactor was constant at 0.1 day−1 throughout the perturbation period. A rapid accumulation in volatile fatty acids (VFAs) occurred immediately after initiating the perturbation. During the first 45 days, effluent acetate, propionate, and butyrate concentrations increased to ca. 2,500, 800, and 1,200 mg l−1, respectively; effluent chemical oxygen demand (COD) increased from 450 mg l−1 to 5,200 mg l−1. Total gas production decreased from about 600 ml day−1 to an average value of 290 ml day−1; CH4 content of the biogas decreased from 50% to about 30%; and the pH decreased from 7.0 to 6.4. This was followed by a 60-day metastable ``steady'' state during which time the effluent COD and VFA concentrations fluctuated about new median values. A dramatic change in the fermentation products of glucose was observed. At the end of the metastable period, VFA concentrations decreased rapidly. First, a rapid decrease in butyrate concentration occurred. Subsequently, acetate and propionate concentrations decreased to near the original preperturbation steady-state levels. Within ca. 30 days, a new steady state was established. These observations demonstrate that the anaerobic system was able to adapt to the periodic substrate perturbation through a long-term change in community structure. This conclusion was verified by comparison of DNA extracts from the steady-state control ``mother'' reactor with extracts from the perturbed ``daughter'' reactor, changes in substrate degradation rates, and microscopic examination.