Mean serum inorganic phosphorus concentrations of cows in an infertile dairy herd of 170 cows in the Hunter Valley increased from less than 4.5 mg/100 ml to greater than 5.8 mg/100 ml after defluorinated superphosphate was added to their drinking water at the rate of 2.5 kg/450 1 once weekly from September, 1976. The first service pregnancy rate increased from 36.5% to 63.2%, the mean calving to conception interval decreased from 109 days to 85 days and the number of cows culled each year for infertility fell from 15 to 5. The pasture fed to the milking cows contained on average greater than 17% crude protein, greater than or equal to 0.39% phosphorus, greater than or equal to 9 ppm copper and 38--46 ppm manganese according to the dominant species. The Ca/P ratio ranged from 2 to 5.5:1. Of these, only the Ca/P ratio and the manganese concentrations appeared to be abnormal.