Previous protein engineering research in this laboratory has yielded an Escherichia coli AppA2 mutant phytase with improved thermostability and an Aspergillus niger PhyA mutant phytase with improved thermostability and pH profile. The objective of this study was to determine the effectiveness of these phytase variants in improving phytate phosphorus utilization by weanling pigs. A total of 40 pigs (5-week old, Yorkshire-Landrace-Hampshire crossbred) were fed a corn-soybean meal based basal diet (BD, without supplemental inorganic phosphorus) or the BD supplemented with the wild-type of AppA2, the AppA2 mutant, or the PhyA mutant at 300 U/kg of diet for four weeks. The phytase activity was assayed in citrate buffer, pH 5.5. Pigs (n = 10/treatment) were housed individually and had ad libitum access to feed and water. Daily feed intake and weekly body weight change of individual pigs were recorded, and blood samples of individual pigs were taken weekly to measure plasma inorganic phosphorus concentrations and alkaline phosphatase activity. Pigs fed the wild-type and the AppA2 mutant phytases had higher (P < 0.01) plasma inorganic phosphorus concentrations from week 2 through week 4 and lower (P < 0.05) plasma alkaline phosphatase activity in week 4 than did pigs fed the BD or the PhyA mutant. The latter two groups showed similar values in these measures. The overall growth performance of pigs was not affected by the dietary treatments. In conclusion, the AppA2 mutant phytase engineered for improved thermostability was as effective as the wild-type in releasing phytate-phosphorus from the diet for weanling pigs. The feeding performance of the PhyA mutant reinforces the difficulty in predicting nutritional value of recombinant enzymes.