This study was conducted to determine the efficacy of Natuphos® phytase for improving the bioavailabilities of phytate P and other nutrients (CP, AA, Ca, and Zn) bound to phytate in the diets for pigs, broilers, and turkey poults, and to evaluate P and Zn equivalency values of phytase. In a 5-wk study with young pigs (BW = 7.5 kg; n = 96) fed a soybean meal-based semi-purified diet (SP), performance, P absorption, and bone characteristics were improved, as graded levels of phytase (0, 350, 700, 1,050, and 1,400 U/kg of diet) were added to the low P diets (.05 and .16% available P, aP). In comparison with the .32% aP diet, fecal P excretion decreased 25 to 50% by adding phytase. The replacement of 1 g P as defluorinated phosphate (DFP) would require about 676 U of Natuphos® phytase. Effect of phytase on Ca and N absorption were variable. Three experiments were conducted with 1,856 broilers (d 1 to 21) fed corn-soybean meal-based diets (CS, Exp. 1 and 2) or SP, (Exp. 2 and 3). A 3 x 7 factorial arrangement of treatments was used in Exp. 1 with .20, .27, and .34% nonphytate P (nP) and 0, 200, 400, 600, 800, 1,000, and 1,200 U phytase/kg of diet. Four graded levels of phytase (0, 350, 700, and 1,050 U/kg of diet) and nP (.27, .36, .45, and .54%) were used in Exp. 2 and 3 with .27% nP as a basal diet. Adding phytase consistently increased BW gain, feed intake, toe ash percentage, and apparent retention of P and Ca. The magnitude of the responses was greater with the lower nP than with the higher nP (Exp. 1). In comparison with the .45% nP diet, P excretion of broilers decreased 25 to 60% by addition of phytase. The average of P equivalency values of phytase from these three experiments indicates that the release of 1 g P as DFP requires 920 and 830 U of Natuphos® phytase for broilers fed SP and CS, respectively. Apparent N retention was increased with addition of phytase in broilers fed CS (only measured in Exp. 3). In a study with turkey poults (n = 480, d 1 to 29) fed CS, a 2 x 2 x 2 factorial arrangement of treatments was used with .45 and .60% nP, 22.5 and 28.0% CP, and 0 and 750 U of phytase/kg of diet. Microbial phytase enhanced apparent and true ileal digestibility of N and AA, apparent retention of N and P, performance and toe ash content. The major improvement of phytase on N and AA digestibility ranged from 1 to 4 percentage units was obtained at .45% nP with 28.0% CP or .60% nP with 22.5% CP. The effect of phytase on the utilization of Zn was determined in broilers (d 1 to 21, n = 384) fed a corn-soy isolate diet (20 ppm Zn). Supplemental phytase (150, 300, 450, and 600 U/kg of diet) improved Zn utilization based on measurements of Zn content in toe, tibia, and liver, apparent Zn rentention and performance. The results indicate that about 1 mg of Zn was released per 100 U of Natuphos® phytase over the range of phytase added. It was observed in pigs that the stomach was the site of highest added phytase activity. In summary, microbial phytase is effective in improving the utilization of P, Ca, Zn, N, and AA in the diets for pigs, broilers, and turkey poults. / Ph. D.