A glasshouse trial with wheat (Triticum aestivum L.) has shown that the effectiveness relative to superphosphate of calcined Christmas Island C-grade rock phosphate applied singly and in mixtures with superphosphate was not dependent on rate of application. There was no evidence of any interaction between the two P fertilizers when they were applied together. The exponential character of the fitted response curves allowed a relative effectiveness value of 0.18 ± 0.03 to be determined from the ratio ofb3 andb4 (curvature coefficients) confirming the poor performance of this rock phosphate fertilizer.