The objective was to evaluate effects of microbial phytase on apparent total tract digestibility (ATTD) of P in a non-heat-treated and a heat-treated wheat (Triticum aestivum)-barley (Hordeum vulgare) diet fed without inorganic P in a 2 × 3 factorial arrangement. The basal diet was ground and half of the batch was steam pelleted at 81°C and crumbled. Phytase was added at 0, 250, and 500 phytase units (FTU)/kg as-fed (Aspergillus niger). The study comprised 36 pigs from 6 litters. Pigs were housed in metabolism crates and fed 1 of 6 diets for 12 d: 5 d for adaptation and 7 d for total collection of feces. The ATTD of P was highest (P < 0.01) for the non-heat-treated diets and highest (P < 0.01) for the phytase-supplemented diets. Heat treatment reduced plant phytase activity by 25% whereby the ATTD of P decreased (P < 0.01) from 57 to 49%. Microbial phytase increased the ATTD of P to a maximum of 64 and 61% in the non-heat-treated and heat-treated diets corresponding to an increase of 7 and 12%-units. Responses for ATTD of P did not differ between 250 vs. 500 FTU/kg as-fed. In conclusion, processing of feed (meal or pellets) containing plant phytase should be considered to avoid over- or underestimation of effects of microbial phytase.