Two experiments were conductephyd to study the effects of phytase derived from Escherichia coil (ECP) and Peniophora lycii (PLP) on performance, nutrient digestibility, and phosphorus (P) equivalency values of young pigs; and the influence of varying ratios of calcium (Ca) to total phosphorus (Ca:tP) with or without ECP on growth performance of pigs. In each experiment, 48 10-kg pigs were housed in individual pens for 28 days. Experiment 1 was designed to study the efficacy of ECP and PLP in improving growth performance and bone mineralization of young pigs given a low inorganic P (iP) diet. In the experiment, pigs were blocked by weight and sex and randomly allocated to eight dietary treatments. Each treatment had six replicates. The treatments were a positive control with adequate iP (PC); a low iP negative control (NC) diet; NC with supplemental iP in the form of monosodium phosphate added to provide 0.75 or 1.50 g iP per kg diet (as-fed basis); NC with ECP or PLP added at 500 or 1000 FTU per kg (as-fed basis; one phytase unit or FTU is defined as the quantity of enzyme required to liberate 1 µmol of iP per min, at pH 5.5, from an excess of 15 µmol/l sodium phytate at 37°C). In experiment 2, the objective was to study the effect of varying Ca:tP ratio with or without ECP on growth performance of growing pigs. Pigs (eight replicates) were blocked by weight and sex and randomly assigned to six dietary treatments in a 3 x 2 factorial arrangement of Ca:tP ratios at 1.2, 1.5, or 1.8; and ECP at 0 or 1000 FTU per kg (as-fed basis). In experiment 1, ECP (P < 0.001) and PLP (P < 0.05) linearly increased daily gain. There was a positive linear (P < 0.05) response to the supplementation of the NC diet with ECP or PLP in the mineralization of the third and fourth metacarpal bones. Phosphorus equivalency values for 500 FTU of ECP or PLP based on mineralization of the third metacarpal bone were 0.77g or 0.572 g, respectively. There were linear (P < 0.001) and quadratic (P < 0.05) responses to ECP, and linear (P < 0.01) response to PLP in P digestibility. Neither iP supplementation nor either of the two phytases had significant effects on digestibility of dry matter, protein or energy. In experiment 2, reducing Ca:tP ratio linearly improved (P < 0.05) daily gain, food intake, and food efficiency in pigs regardless of phytase supplementation. There was an enzyme by Ca:tP interaction on food intake. These studies in young pigs showed that ECP and PLP were efficacious in improving the growth performance and bone mineralization and that reducing the Ca:tP ratio enhanced performance response to phytase supplementation. [ABSTRACT FROM AUTHOR]