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Italian ryegrass (Loliwn multiflorum cv. Midmar) responses to fertilizer P were studied in field experiments under irrigation on highly?weathered ferrallitic soils at two localities in Natal. Each experiment was conducted over two seasons (1985 and 1986), the pasture being re?established after the first season. Large dry matter yield responses to P were measured on both soils. Phosphorus effects were greatest during the establishment phase (autumn) and, on the colder site, in spring. Concentrations of P in herbage were much lower in winter than in autumn or summer. The greater responsiveness to P in spring and the lower P concentrations in herbage during the colder months suggested that restricted P uptake contributed to yield reductions in winter. Relationships between ryegrass P concentrations and yield indicated substantial within?season variations in internal P requirement for maximum yield. This detracts from the usefulness of routine herbage P analyses for assessing ryegrass P sufficiency. However, soil analysis using an alkaline extractant (NH4HCO3/EDTA/NH4F at pH 8.0), showed promise for predicting responsiveness to P. Concentrations of P in Italian ryegrass were well below the requirements reported in the literature for most classes of grazing ruminants, particularly during the colder months.