The impact of modified mineral status and dietary Ca:P ratio on Ca and P utilisation was measured in chicks with or without phytase supplementation. In a preliminary study, 4 diets were given to chicks from 3 to 15 d of age: D1 (65 g P/kg and Ca:P = 15) and D2, D3 and D4 (60, 54 and 50 g P/kg, respectively, and Ca:P = 12). Growth performance was similar across diets. Tibia ash was similar in chicks given D1 and D2, but was gradually depressed from D2 to D4 (-22%). In the depletion period, two groups of chicks, with similar performance, but with different mineral status were achieved by feeding them, from 5 to 15 d of age, diets with a similar Ca:P ratio of 12, but containing 63 or 52 g P/kg. During the subsequent 11 d of the repletion period, chicks from each of the two previous groups were given one of the 4 diets containing 57 g P/kg, but differing in their Ca (83 and 53 g Ca/kg) and microbial phytase (0 or 1000 FTU, Natuphos) levels in a 2 2 2 factorial arrangement. At the end of the repletion period, the initially depleted chicks could not be differentiated from the non-depleted chicks, indicating the capacity of chicks to compensate for their initial depleted mineral status. Interaction between dietary Ca and phytase levels was not significant. Phytase improved growth performance and bone characteristics. Reduced dietary Ca enhanced feed intake and growth rate, but depressed bone dry matter and ash weight. At the end, diets supplemented with phytase maximised bone ash weight when chicks were fed with a Ca:P ratio of 15 but elicited the highest growth rate when chicks were fed with a Ca:P ratio of 09.