A total of 364 female Ross 308 chicks (1 d old) were used in the present study conducted in floor pens to investigate the effects of graded levels of supplementary bacterial phytase on dietary energy utilisation and growth performance. For this purpose, four maize–soyabean-based diets were offered to the birds from 0 to 21 d of age. These included a suboptimal P negative control (NC, 3·0 g/kg non-phytate P), NC+250 phytase units (FTU)/kg feed, NC+500 FTU and NC+2500 FTU. The effect of phytase activity on bird growth performance was best described as a linear relationship between increasing dose and increased feed intake (P< 0·001), but was quadratic for body-weight gain (P= 0·002) and feed efficiency (P= 0·023). There was no significant response (P>0·05) of dietary apparent metabolisable energy (AME) to supplementary phytase. The birds fed phytase increased their retention of total carcass energy in a linear fashion (P= 0·009) with increased phytase dose. The efficiency of dietary AME used for overall carcass energy retention also improved (P= 0·007) in a linear manner with increased dietary phytase activity. Dietary net energy for production (NEp) increased (P= 0·047) with an increase in phytase dose following a linear pattern, as an increase of 100 FTU increased dietary net energy by 15·4 J (estimated within the range of doses used in the present experiment). Dietary NEp was more highly correlated with performance criteria than dietary AME, and it seems to be a more sensitive way to evaluate broiler response to phytase supplementation.