Dry matter disappearance (DMD) and cellulose disappearance (CD) from alfalfa hay were measured simultaneously in synthetic fiber bags incubated in vitro. In general, DMD, CD, and pH were reduced (P less than .05) inside the bags compared with values in the fermentation flasks. Bags became fully expanded with gas soon after the fermentation started, which seemed to inhibit fluid movement into and out of the bag. By creating a vacuum in the flask, fluid exchange was enhanced, and DMD, CD, and pH increased (P less than .01) inside the bags. Sample size, type of fabric, and fabric pore size affected pH, DMD, and CD. Inoculum source (forage or concentrate diet) did not affect DMD in either bags or flasks. In contrast, DMD in bags incubated in vivo was affected (P less than .01) by animal diet. When bags were incubated in vivo inside a rigid plastic container, both pH and DMD were reduced (P less than .01) compared with bags suspended freely in the rumen. Direct physical action of ruminal contents on the bags apparently enhanced exchange of fluid. Bacterial concentrations in bags suspended freely in the rumen were greater than those in bags incubated inside containers. Synthetic fiber bags may be useful in comparing digestibility between diets; however, caution is needed in evaluating such data in absolute terms.