Reports about the impact of supplemental phytase on metabolisable energy (ME) have been inconsistent. However, ME does not consider partitioning of energy for maintenance, product, or heat increment. Net energy (NE) is a more meaningful measure to evaluate the response of broilers to phytase application in this regard. To date, few data can be found on the effect of microbial phytase on dietary net energy. The current study assessed the impact of three phytase products supplemented at an unconventional high level (all at the inclusion rate of 1000 FTU / kg feed) on net energy of broilers using the indirect calorimetric method (IC). The four treatments were: control, which is a wheat-soybean meal (SBM) based diet with reduced available phosphorus (aP) (2.0 g/kg) and calcium (Ca) (6.0 g/kg); control + intrinsically thermostable 'E. coli' phytase A (phytase A); control + intrinsically thermostable 'E. coli' phytase B (phytase B); control + coated 'E. coli' phytase (phytase C). On d 21, 32 male broilers (Ross 308) were allocated to 16 closed-circuit chambers. The adaptation period was four days and then heat production was calculated by applying the Brouwer equation to measurements of O2 consumption and CO2 production from 25 d to 27 d. ME was determined by the total collection method and NE was calculated as fasting heat production (450 kJ/BW 0.70) + energy gain (Noblet et al., 2010).