The experiment was carried out on 216 Hisex brown hens, aged 48-70 weeks, divided into 12 groups of 18 birds kept in single cages. Layers were fed diets containing 0.45, 0.52 and 0.58% total P (0.25, 0.30, 0.35% available P) supplemented with 0, 150, 300 or 450 units of microbial phytase (FTU) per kg. At 54, 62 and 70 weeks of age, eggs and egg-shell quality were analysed. Level of P and phytase had no effect on feed intake, laying rate, yolk percentage and albumen:yolk ratio. Eggs were heaviest (65.1 g) when no phytase was added or when the diet containing the lowest P level was fed (P ≤ 0.01). Specific gravity, shell breaking strength, shell thickness, shell percentage and shell index (density) were highest when the diets were supplemented with 150 FTU/kg, regardless of the P level. However, differences were significant (P ≤ 0.05) only for shell breaking strength and thickness. On the diets supplemented with 450 FTU, some parameters of egg-shell quality were worse than without the addition of microbial phytase. However, a much smaller number of cracked and broken eggs was found when phytase was added.