The effect of cow and in situ bag type on undigested NDF (uNDF) content of corn silage, wheat straw, and alfalfa silage after 240 h in situ incubation were evaluated in three rumen-cannulated Holstein cows. Two corn silages, a wheat straw, and an alfalfa silage sample were dried and ground to pass through a 2.5-mm screen. After thorough mixing, 1 , 2, and 5 g of each forage were placed into F57 Ankom bags (4 by 5 cm), Ankom 5- by 10-cm nylon bags (part number R0510), or Ankom 10- by 20-cm nylon bags (part number R1020), respectively. There were three duplicates for each treatment within each cow. Sample mass to surface area by bag was 25, 20, and 12.5 mg cm−2, respectively. The residual NDF was analyzed after 240 h incubation. Data was analyzed by SPSS 19.0. Within each forage, the effect of cow and bag type on uNDF within forages was determined by ANOVA. Means were compared by Duncan's multiple range test. The contents of the uNDF significantly differed due to bag type (P < 0.001). The content of uNDF in F57 bags was significantly higher than in R0510 and R1020 nylon bags (P < 0.05) and there was no significant difference in uNDF between R0510 and R1020 nylon bags (P > 0.05). The uNDF differed within cow for wheat straw (P < 0.001). Bag type and effective surface area should be taken into consideration when measuring uNDF by in situ methods; the estimate of uNDF in relatively indigestible materials, such as wheat straw, may also be affected by cow.