An experiment was undertaken to determine the effect of the ratio of calcium to total phosphorus in the diet on the effectiveness of the phytase enzyme, and also to determine the effect of the calcium:phosphorus ratio on the amount and form (soluble versus insoluble) of phosphorus excreted when the diet contains phytase. The use of phytase in pig diets is rapidly increasing as extensive research has documented its efficacy in improving the digestibility of phosphorus in cereal grains. This allows diets to be formulated with less total phosphorus, resulting in a reduced cost of feed and decreased phosphorus output in the manure. This will become even more important if and when environmental farm plans are based on phosphorus output in the manure rather than nitrogen, or in addition to nitrogen. It is also well known that as the animal’s intake of phosphorus declines, the calcium:phosphorus ratio becomes increasingly important. It was concluded that the differences in phosphorus output, due to either phytase or calcium:phosphorus ratio, are largely due to changes in the soluble inorganic fraction – the largest phosphorus fraction and the fraction which is thought to be responsible for problems of leaching. The effective use of phytase requires careful attention to the ratio of calcium to total phosphorus in the diet. Also, adding phytase to a swine diet is favourable environmentally, because it decreases output of both total and soluble phosphorus.