The relative bioavailability values of P from seven sources were determined using male chickens fed a basal corn-soybean meal diet (.40% total P and 1.10% Ca) with varying levels of test and standard phosphates from 0 to 3 wk of age. Each of seven test phosphates was added to the basal diet at .05, .08, .12, .17, .23, and .32% levels. In addition, standard diets were formulated by adding the same levels of P plus .44 and .60% levels from dicalcium phosphate dihydrate (CaHPO4.2H2O) to the same basal diet. Each of the 42 test diets was fed to two pens of 10 chicks each, and each of the standard diets was fed to six pens of 10 chicks each. Three-week body weight gain and percentage ash of dried toe data were fitted with nonlinear (asymptotic and sigmoidal) regression equations, and the ratios of regression coefficients were used to determine the bioavailability of various test phosphates relative to the reference standard. Based on the asymptotic regression equation of body weight gain and on the sigmoidal regression equation of toe ash on the percentage of P added to the diets from the seven phosphate sources, the average relative bioavailability of the P (compared with the standard given a value of 100%) was as follows: Lucaphos-48, 88.4%; Lucaphos-40, 95.1%; Rukana, 83.7%; Cefkaphos-N, 104.8%; phosphoric acid, 93.0%; monocalcium phosphate monohydrate [Ca(H2PO4)2.H2O], 111.8%; and Biophos, 92.0%. A difference of 7.7% units for the average bioavailability of the P was required for significance (P < .05).(ABSTRACT TRUNCATED AT 250 WORDS)