Santos, T. T., Walk, C. L., Wilcock, P., Cordero, G. and Chewning, J. 2014. Performance and bone characteristics of growing pigs fed diets marginally deficient in available phosphorus and a novel microbial phytase. Can. J. Anim. Sci. 94: 493-497. The efficacy of a novel microbial 6-phytase on pig performance, bone breaking strength, and bone ash was evaluated. Pigs were allotted to one of six diets with 12 replicate pens/treatment and five pigs/pen. Diets consisted of a positive control (PC), negative control (NC) 1 with Ca and available phosphorus (avP) reduction by 0.11 and 0.10%, NC 1 plus 250 FTU kg^-1 phytase, NC 2 with Ca and avP reduction by 0.16 and 0.15%, NC 2 plus 500 and 2,000 FTU kg^-1 phytase. On day 43, one pig per pen was euthanized and the 4th metacarpal was obtained to determine bone parameters. Average daily gain (ADG) was higher (P<0.05) in pigs fed NC 2 plus 500 or 2000 FTU kg^-1 compared with PC. NC 2 with 2000 FTU kg^-1 improved (P<0.05) G:F compared with PC. NC 1 or NC 2 had reduced (P<0.05) bone ash weight and NC 2 had reduced (P<0.05) bone breaking strength compared with PC. Phytase supplementation in NC 1 or NC 2 improved bone ash weight compared with PC. Bone breaking strength or bone ash weight was more sensitive to low dietary Ca and avP than bone ash percent. Phytase supplementation at 2000 FTU kg^-1 improved G:F beyond that of the PC.