Iron pyrites (FeS2), without any special treatment other than grinding, was found to be highly effective as a sulphur fertilizer for Trifolium subterraneum L. The dry weights of tops of plants fertilized with iron pyrites at 170 lb/acre (50% by weight of particles < 9 µ in diameter) or at 340 lb/acre (14% < 9 µ in diameter) were more than three times those of plants which did not receive applied sulphur (P < 0.001), and were similar to those of plants receiving 40 lb sulphur per acre (sulphur sufficiency) as either sodium sulphate or sublimed sulphur. The smallest mean particle size used (65% < 9 µ in diameter) did not produce any further increases in dry weights. Iron pyrites at very high rates of application (1360 and 680 lb/acre) of the smallest mean particle size were not toxic to the plants. Small quantities of sublimed sulphur applied with the iron pyrites did not increase the dry weight responses to the iron pyrites. Sterilization of the iron pyrites, or inoculation of sterilized iron pyrites with either an American or a Tasmanian culture of iron-oxidizing bacteria similar to Thiobacillus ferrooxidans, had no effect on the dry weight responses. No indications were found that the presence of phosphate ions decreased the dry weight responses to iron pyrites. It is suggested that in warmer humid climates, smaller applications or larger mean particle sizes of iron pyrites might suffice to alleviate a similar degree of sulphur deficiency.