Calcium is an important cation in chicken diets, being the most abundant element in the body of the chicken. Its functions include: mineralization of bones, blood clotting, enzyme activation, neuromuscular function, muscle contraction, and intracellular signaling. However, high dietary Ca is a limiting factor for phytase efficacy and the formation of insoluble Ca-phytate complexes decreases Ca and P availability. Furthermore, Williams (2005) has shown high dietary Ca concentration in the gastrointestinal tract to be associated with pathogenesis of necrotic enteritis (NE). This study investigated the hypothesis that high dietary calcium would decrease phytase efficacy and decrease performance in chickens either challenged or unchallenged with NE. Ross 308 male broiler breeder (n=768) were randomly distributed to 8 treatments in a factorial arrangement. Factors were: calcium level (0.6 or 1.0%), phytase level (500 or 1500 FTU/kg) (Quantum Blue^TM, AB Vista, Malborough, UK) and NE challenge (no or yes). There were 48 pens, 16 birds per pen and 6 replicates per treatment. Performance was measured weekly with the exception of feed intake (FI) and feed conversion ratio (FCR) which were not measured on d 21. The FCR was corrected for mortality (FCRc). Half of the birds (384) were challenged with 5000 unattenuated sporulated oocysts each of <i>E. acervulina, E. brunetti</i> and <i>E. maxima</i> (Bioproperties Pty Ltd) on d 9, and 10⁸ CFU per mL of <i>C. perfringens</i> strain EHE-NE18 (known to express NetB toxin, CSIRO) on d 14 and again on d 15.