In the mid-1990s Ankom Technology developed a filter bag system for fiber determination. This system reduced labor and costs and simplified the operation. More recently, a new filter bag (F-58) was developed. This bag possesses reduced pore size and has been directed toward end users who utilize a finer grind size to support near infrared spectroscopy (NIRS). Little work has compared the older (F-57) and newer (F-58) bags. The primary objective of this study was to compare F-57 and F-58 filter bags for determination of ADF and NDF. Four forage species were collected, dried, and sequentially ground to pass through a 2 and 1 mm screen. These species included orchardgrass (Dactylis glomerata L.), bermudagrass (Cynodon dactylon L.), white clover (Trifolium repens L.), and sericea lespedeza (Lespedeza cuneata L.). Acid and NDF were determined using the Ankom 200 Fiber Analyzer. Trial x treatment interactions occurred for ADF (P < 0.01). Therefore data are presented by trial. Within trials, significant forage x filter bag interactions occurred (P < 0.01). In general, the use of the F-58 bag decreased ADF concentrations in all forages except bermudagrass (P < 0.01) and had no effect on NDF concentrations (P > 0.05). However, the impact on ADF concentrations was not consistent across forage species. Bermudagrass ADF concentrations did not differ between bags for either trial and orchardgrass differed in Trial I only where the % ADF was decreased from 38 to 35% for F-57 and F-58 bags respectively (P < 0.01). At the current time our data indicate that the inconsistent performance across forage species and the increased cost does not justify the use of the F-58 bags.