Phytase is added to swine diets to improve the utilization of phytate-bound P. This provides financial and environmental benefits to the pig industry. It is unclear if phytase works equally well under all dietary circumstances. The objective was to determine if insoluble fiber impacts the efficacy of the phytase enzyme in nursery pigs when fed diets limiting in P content. A total of 480 pigs (initial BW 5.48 ± 0.14 kg) were blocked by BW and randomly assigned to treatment within block. A common nutrient-adequate diet was fed from d -14 to -5, and 2 basal P deficient diets (either a corn-soy diet containing 0.16% standardized total tract digestible [STTD] P [LF], or a corn-soybean meal plus 20% corn bran containing 0.14% STTD P [HF]) were fed from d -5 to 0 to acclimate pigs to a P deficient diet. From d 0-21, pigs received 8 dietary treatments (6 pens per treatment: n=6). Experimental diets consisted of LF supplemented with one of 4 levels of added phytase (0, 109, 218, and 327 phytase units [FTU]/kg; Quantum Blue 5 G, AB Vista, Wiltshire, UK) expected to provide 0.16, 0.21, 0.26, and 0.31% STTD P, respectively, or HF supplemented with one of the same 4 levels of added phytase. Titanium dioxide was added to the diet at 0.4% as an indigestible marker. On d 21, one pig representing the average BW for each pen was euthanized, and fibulae were collected and analyzed for bone ash. Fecal samples were collected from each pen on d 19-20. Data were analyzed using PROC MIXED of SAS. There were no interactions between insoluble fiber and phytase for any of the variables evaluated. For d 0 - 21, adding phytase increased ADG (P < 0.001) with the response being linear (P < 0.001), whereas insoluble fiber decreased ADG (P = 0.033). There were no effects of phytase or insoluble fiber on ADFI (P = 0.381 and P = 0.632, respectively). Phytase improved G:F ratio (P < 0.001) with the response being linear (P < 0.001). Insoluble fiber tended to decrease G:F ratio (P = 0.097). Phytase increased bone ash (P = 0.005) with the response being linear (P = 0.001), but there was no effect of insoluble fiber (P = 0.949). Phytase did not affect the apparent total tract digestibility of DM, NDF, or ADF (P > 0.050), whereas insoluble fiber decreased the ATTD of DM (P < 0.001), NDF (P < 0.001), and ADF (P < 0.001). In conclusion, the addition of insoluble fiber did not affect the ability of phytase to improve growth performance and bone mineralization in nursery pigs fed a P deficient diet.