A total of 936 one-day-old broiler chicks (Ross 308) was used to evaluate the effects of dietary calcium (Ca) source (1.0 and 0.6% from Ca carbonate [CC], or 0.6% from Celtic sea minerals [CSM]) on broiler chick response to phytase supplementation (0, 500, or 2,500 FTU per kg of diet) in phosphorus (P) adequate diets. Birds were randomly assigned to 9 treatments, each replicated 8 times (4 repeats of male and 4 repeats of female birds; 13 birds per replicate) in a completely randomized design. Results showed that birds fed low Ca CC diets had significantly (P ≤ 0.05) lower body weight at 14 and 21 d; weight gain during 1 to 14 d; feed intake during 1 to 14 d and 1 to 28 d; and toe and tibia ash content, ileum ash, and Ca, and P contents at 28 days. Feed conversion ratio and feed intake between 21 and 28 d, and serum Ca, Fe, and alkaline phosphatase levels at 28 d of age were not affected by dietary Ca level. The negative influence of reducing the dietary Ca level on body weight and weight gain was less severe when CC was replaced with CSM, and birds fed the CSM diet had a significantly lower feed conversion ratio and higher tibia P content compared to CC. Phytase did not have a significant influence (P > 0.05) on most measured parameters, but significantly reduced ileum P and ash contents, especially at the higher level of 2,500 vs. 500 units of phytase per kg of diet. These results confirm that CSM has a valuable potential to improve feed conversion ratio, and that phytase was less effective in compensating for a significantly reduced dietary Ca level as it is for P.