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A digestibility experiment using six ileally cannulated growing gilts (initial BW 31.6 kg) was carried out to study the effect of microbial phytase and diet form on apparent ileal and total tract digestibilities of dry matter (DM), nitrogen (N), phosphorus (P), calcium (Ca) and energy and on apparent ileal digestibility of amino acids. The basal P-deficient diet was fed either dry or mixed with water (feed:water ratio 1:2.5) and fermented in a laboratory setup. The enzyme was added to the dry diet at four levels (0, 1 000, 1 500 and 3 000 U/kg) and to the liquid fermented diet at two levels (0 and 1 000 U/kg) using a 6 × 6 Latin square design. The supplementation of microbial phytase to the basal diets significantly (P < 0.05) improved ileal and total tract digestibility of P and tended to improve the digestibility of Ca. Phytase supplementation at 3 000 U/kg to the dry diet improved ileal and total tract P digestibility by approximately 30 and 60%, respectively. A similar effect was found in the liquid fermented diet supplemented with phytase at 1 000 U/kg. As a result of improved P digestibility, faecal P excretion was reduced by 20-40%. There was no significant effect of phytase addition or diet fermentation on the digestibility of DM, N, energy or amino acids. The response in amino acid digestibility to phytase calculated with acid insoluble ash as a marker was slightly higher than that calculated with Cr2O3.