An 18-d experiment was conducted to determine the effect of varying mix uniformity of phytase on growth performance, mineral retention, and bone mineralization in chicks. Chicks (initial and final weights were 74.5 and 803.3 g) were allotted to seven treatments with six (Treatment 1) or seven (Treatments 2 to 7) replicates of seven chicks per replicate in a completely randomized design. Varying mix uniformity of phytase was simulated by alternately providing two diets with two different concentrations of microbial phytase; the diets were switched every 24 h. Treatments were: 1) positive control (CON) (Ca, 1.0%; available phosphorus (aP), 0.45%), 2) negative control (NEG) (Ca, 0.9%; aP, 0.35%), 3) NEG + 600 phytase units (FTU) daily (CV0), 4) NEG + 500 or 700 FTU (CV17), 5) NEG + 400 or 800 FTU (CV34), 6) NEG + 200 or 1,000 FTU (CV69), or 7) NEG + 0 or 1,200 FTU (CV103). Gain, feed intake, and bone breaking strength were similar (P > 0.15) in the CON and CV0 treatments, but these response variables were decreased in the NEG treatment (P < 0.01). Gain:feed was not affected by treatment (P = 0.15). Bone ash was decreased (P < 0.02) by the NEG and CV0 treatments compared with the CON diet, but chicks fed the CV0 diet had greater bone ash than those fed the NEG (P < 0.01) diet. Increasing FTU CV decreased bone breaking strength and bone ash (P < 0.01). Calcium and phosphorus retention (P < 0.08) and gain (P < 0.09) were numerically decreased, and phosphorus excretion was numerically increased (P < 0.07) as FTU CV increased. The difference between the CV0 and CV103 treatments was significant only for bone breaking strength and ash (P < 0.01). In conclusion, increasing phytase CV had little effect on growth performance, whereas bone ash and breaking strength and calcium and phosphorus retention and excretion decreased only at the most extreme CV.