Recent meta-analyses on feeding palmitic fatty acid (PFA) supplements to dairy cattle indicate consistent improvement in total-tract NDF digestibility. However, the mode of action for increased NDF digestibility remains unexplained. Our hypothesis was that PFA supplementation increases fiber digestibility within the rumen. In the present study, we fed either a control (CON) or a palmitic fatty acid (PFA) diet to dual-flow continuous culture fermentors (n = 8, 1.82L) to evaluate the effect of PFA supplementation on NDF digestibility and VFA production when PFA was supplemented rather than substituted for dietary NDF. The CON diet was a 50:50 forage:concentrate pellet mixture that provided 7.0 g CP, 14.6 g NDF, and 4.9 g starch at 37.2 g DM once daily with no supplemental fat (1.3 g EE/d). The PFA diet was a 50:50 forage:concentrate pellet mixture that provided 6.6 g CP, 14.5 g NDF, 5.2 g starch, and 1.1 g PFA at 37.6 g DM once daily (1.9 g EE/d). Two experimental periods lasted 11 d each, with 4 d of sample collection. Buffer dilution and solids passage rate were maintained at 7.0%/h and 5.0%/h, respectively. Data were analyzed using a mixed model including the fixed effect of treatment, the random effects of fermentor and period, and a repeated statement for sampling day. There was no effect (P > 0.2) of treatment on apparent organic matter digestibility. Feeding PFA decreased (P < 0.01) NDF digestibility by 2.72 percentage units. There was no effect (P = 0.19) of PFA on daily VFA production nor was there an effect (P ≥ 0.12) of PFA on molar proportions of acetate, propionate, isobutyrate, valerate, or isovalerate. However, PFA increased (P = 0.04) butyrate molar proportion compared with the control (0.130 versus 0.124, respectively, SE: 0.002). Feeding PFA tended to decrease (P = 0.07) fermentor ammonia concentration by 1.08 mg/dL. These results contrast with published in vivo evidence of PFA improvement of NDF digestibility and indicate that the rumen microbial adaptation is an unlikely source for the effect of PFA on NDF digestibility. Future research should focus on other factors potentially affecting NDF digestibility when PFA is fed, including feed intake behavior and rumen retention time.