WALDROUP et al. (1965) pointed out that variations in the calcium content of the diet used for assaying phosphorus sources may significantly influence the relative biological value of a particular source. In this report it was suggested that the use of a specific calcium level at different phosphorus levels was desirable in order to elicit maximum response of the chick and allow full utilization of a particular phosphorus source. Harms et al. (1967) suggested calcium levels to be used with various suboptimal levels of phosphorus supplied by monosodium phosphate. In a later report by Damron and Harms (1968) ideal calcium levels were determined for various levels of phosphorus supplied by soft phosphate, defluorinated phosphate, and calcium phosphate. The experiment reported herein was conducted to determine the ideal calcium levels to be used when the supplemental phosphorus was supplied by Curacao Island phosphate. EXPERIMENTAL PROCEDURE In each of three trials, three . . .