Defluorination of rock phosphate by heat treatment is an important method of producing phosphates suitable for animal feed. A variety of such products have been evaluated as phosphorus sources for ruminants, e.g. Beeson et al. (1945) recorded that a material produced at a low temperature (600°C) was unsatisfactory for steers, but Ammerman et al. (1957) indicated that a calcined calcium phosphate acidified with either sulphuric or phosphoric acids was fully as satisfactory as dicalcium phosphate for steers, but not for growing sheep. Wise, Wentworth & Smith (1961) and O'Donovan et al. (1965) recorded that defluorinated rock phosphates (produced by unspecified methods) were as useful as dicalcium phosphate for growing cattle. Arrington et al. (1963) have reported that 32P contained in reagent grade dicalcium phosphate was better retained by growing cattle than when present as a defluorinated rock phosphate.