A feeding trial was performed using 4 x 60 day-old chickens (Ross 208 cockerels) raised up to 42 days of age to determine whether exogenous phytase addition increases phosphorus utilisation by broiler chickens, and to assess its effects on some production traits as well as on the ash content and mechanical stability of the tibia. The chickens' feed consisted of maize, wheat, soybean meal, fish meal, yeast, and fat powder. The basic feed was supplemented with inorganic phosphorus in groups A and B. In groups C and D, the amount of the inorganic phosphorus supplement (DCP) was decreased by 50%, at the same calcium/phosphorus ratio. The 50% reduction of inorganic phosphorus supplementation represents a 20% decrease of total phosphorus. To the diets of groups B and D a phytase enzyme preparation (Phytase Novo CT) was added. The calculated exogenous phytase activity was 600 FYT/kg feed. The decrease of inorganic phosphorus did not cause significant differences in the daily weight gain but lowered the feed conversion rate by 10%. Calcium and phosphorus excretion decreased by 18% and 15%, and the breaking strength of the tibia was also lower. Phytase supplementation of the feed at a lower rate of inorganic phosphorus supplementation did not cause changes in the body weight gain but improved the feed conversion rate by 5.6%. Phosphorus and calcium output decreased by 21% and 11%, respectively, but chemical composition and mechanical stability of the tibia were unaltered.