The objective of this experiment was to determine the effects of feeding a high rumen undegradable protein (RUP) diet when dietary fiber level is manipulated within differing forage to concentrate ratio (F:C) on nutrient utilization of precision-fed dairy heifers. Six rumen cannulated Holstein heifers (555.4 ± 31.4 kg BW) were randomly assigned to 2 levels of concentrate, HC [55%] and LC [40%] and to a fiber level sequence [100% oat hay and silage OA, 0% wheat straw WS (low fiber); 83% OA, 17% WS (medium fiber); and 67% OA, 33% WS (high fiber)] administered according to a split-plot 3 × 3 Latin square design (21-d periods). Similar levels of N intake (1.70 g N/kg BW^0.75) and RUP (55% of CP) were provided. All dependent variables were analyzed as a 3 × 3 Latin square using the PROC MIXED procedure of SAS. No differences were observed for dry matter (DM), neutral detergent fiber (NDF), and acid detergent fiber (ADF) apparent digestibility (AD) between HC and LC-fed heifers. Heifers receiving HC diets had greater starch AD (P < 0.01) than LC heifers. Increasing the fiber level through WS addition resulted in a linear reduction of organic matter AD (P = 0.05). There was a linear interaction for DM AD (P = 0.05) with a concurrent linear interaction in NDF AD. Nitrogen intake, AD, and retention were not different among treatments, however, urine N excretion increased linearly (P = 0.03) with added fiber. Uric acid excretion differed among F:C treatments (P < 0.01) but total purine derivatives (PD), microbial CP, and protozoa did not differ between F:C treatments. Microbial CP flow and PD quadratically increased (P < 0.05) with WS inclusion. Mean rumen fluid pH quadratically increased (P = 0.04) with WS addition, although no F:C effect was detected. Ruminal ammonia concentration did not differ among treatments. Apparent digestibility, purine excretion, and ruminal pH were affected differently as dietary fiber was added through the addition of WS in the diet, suggesting that high RUP can have differential effects on nutrient utilization as dietary fiber is manipulated in precision-fed dairy heifers.