Abstract The objective of this experiment was to determine the effect of increased microbial phytase on total tract digestibility of P in feed phosphates when fed to growing pigs. A source of monocalcium phosphate (MCP), monosodium phosphate (MSP), and magnesium phosphate (MgP) were procured. Three basal diets were formulated to contain 0, 500, and 4,000 units of microbial phytase (FTU) and no addition of feed phosphates. Nine phosphate-containing diets were formulated by adding each of the 3 feed phosphates and 3 levels of phytase. A P-free diet was formulated to determine the basal endogenous P loss. A total of 117 growing barrows (BW = 15.56 kg; SD = 1.68) were divided into 3 blocks of 39 pigs and allotted to the 13 diets for a total of 9 pigs per diet. During the experiment, pigs were housed individually in metabolism crates equipped with a slatted floor, a feeder, and a nipple drinker. Fecal samples were quantitatively collected for 4 days after 5 days of adaptation using the marker-to-marker approach. Fecal samples were dried and ground before analysis. Fecal samples, feed ingredients, feed phosphates, and diet samples were analyzed for P, Ca, and dry matter and the apparent total tract digestibility (ATTD) and standardized total tract digestibility (STTD) were calculated. The ATTD and STTD of P in the 3 feed phosphates were calculated using difference procedure. The statistical model included the level of phytase and the P source as the fixed effects and the random effects of period and replicate within period. Contrast statements were used to test for linear and quadratic effects of increasing phytase in diets and the model included diet as the fixed effect and period and replicate as the random effects. Pig was the experimental unit. Results indicated that the ATTD and STTD of P in all diets increased (P < 0.05) with the inclusion of 500 or 4,000 FTU, but the ATTD and STTD of P in feed phosphates were not affected by the inclusion of phytase (Table 1). This indicates that the increases in ATTD and STTD of P that were observed in the mixed diets when phytase was used were due to the release of P from phytate in corn and soybean meal and not from an increase in digestibility of P in feed phosphates. However, MgP had a lower (P < 0.05) STTD of P than MCP and MSP. In conclusion, microbial phytase does not increase the digestibility of P in MCP, MSP, or MgP, but the digestibility of P in MgP is less than in MCP and MSP.