Abstract: The objective of the present experiment was to determine the effects of phytase supplementation on the performance and carcass traits of finishing pigs fed diets containing normal hulled or hulless barley or a newly developed low-phytate hulless barley all formulated without a source of inorganic phosphorus. A total of 72 crossbred pigs weighing an average of 67.9�8.9 kg were assigned on the basis of sex, weight and litter to one of eight dietary treatments in a factorial design experiment. A positive control diet, based on Harrington barley, was formulated to meet the pig’s requirements for total phosphorus. Three experimental diets were formulated based on either Harrington barley (0.28% phytate phosphorus) or the hulless barleys CDC Freedom (0.31% phytate phosphorus) and LP 422H (0.16% phytate phosphorus). The experimental diets were deliberately formulated to be below requirements for total phosphorus by removing all of the inorganic phosphorus (i.e. dicalcium phosphate) from the diet so that the diets contained only organic sources of phosphorus. All four diets were fed with and without 1000 FTU/kg phytase (Natuphos 5000). The addition of phytase tended to improve weight gain (p=0.07) and feed intake (p=0.11). Phytase had the greatest effect in the Harrington diet formulated without dicalcium phosphate improving both daily gain (1.01 vs. 1.16 kg/day) and feed conversion (3.07 vs. 2.83). For the barley diets formulated without dicalcium phosphate, daily gain averaged 1.08, 1.11 and 1.14 kg/day while feed conversion averaged 2.95, 2.72 and 2.74 for the Harrington, CDC Freedom and LP 422H diets, respectively. Neither phytase supplementation nor type of barley had any affect on slaughter weight, carcass weight, dressing percentage or carcass value index. Pigs fed the LP 422H diet without phytase had a significantly higher lean yield (p=0.03) and lower loin fat (p=0.07) than pigs fed the LP 422H supplemented with phytase. The overall results of this experiment indicate that the performance of pigs fed low-phosphorus diets containing phytase is generally improved over unsupplemented diets. In addition, the performance of pigs fed hulless barley-based diets formulated without inorganic phosphorus is superior to that of pigs fed hulled barley-based diets formulated without inorganic phosphorus. Finally, the performance of pigs fed low-phytate hulless barley formulated without a source of inorganic phosphorus but supplemented with phytase is at least equal to that of pigs fed diets containing normal-phytate barley supplemented with inorganic phosphorus. Since inorganic phosphorus sources tend to be expensive, a reduction in their use could lower ration costs thereby increasing the potential profitability of swine production.