The effects of phytase supplementation on the nutritional value, P availability and thyroid status of birds fed 4 differently processed rapeseed expeller cakes (RC) were determined. Two experiments were performed: a balance experiment on 10 groups of nine 3-week-old broilers and a growth experiment on 9 groups of one-day-old broiler females, 11 birds per group. In the balance experiment, a basal (B) and 4 test diets (B mixed with RC in a 6:4 proportion with DM) were fed; half of each diet was supplemented with 1000 U phytase/kg. In the growth experiment, the control diet without RC and isonitrogenous diets with RC supplemented or unsupplemented with 1000 U phytase/kg (RCP) were fed. The amount of RC was 100 or 150 g/kg in diets fed between days 1-21 and 22-42 of life, respectively. The levels of Ca and available P in RC diets were as in the control diet, while in RCP diets they were lowered by 7 and 12%, respectively. Phytase supplementation increased apparent protein digestibility and metabolizable energy values, while it did not affect P availability from RC. BWG was lower in some groups and FCR 5% worse (P<0.05) in all RC groups than in the control. The type of RC significantly affected feed intake, BWG, thyroid and kidney weight. Addition of phytase to RCP diets increased thyroid weight (P<0.01). Phytase addition did not fully compensate for lower Ca and P levels in RCP diets, as tibia ash content was reduced (P<0.05), but this had no effect on tibia weight and ultimate strength.