Seven plant species, barley, cucumber, maize, oats, rice, tomato and wheat were grown in sand for between 9 and 11 weeks and received either 1.33 mM-NaH2PO4 in a complete nutrient solution (P2), or 0.13 mM-NaH2PO4 for 7 days then no added phosphate in an otherwise complete nutrient solution (P1). At harvest plants grown in the P1 solution were much smaller than in the P2 solution, the rice showing the least effect, but only the leaves of cucumber and tomato developed characteristic symptoms of phosphorus deficiency. Acid phosphatase activity was significantly greater in extracts from the leaves of all species grown in the P1, except for rice, when compared with those grown in the P2 solution. The increase was greatest in the leaves of tomato and cucumber which also contained the lowest levels of phosphorus. Acid phosphatase activity, therefore, may be useful as a rapid indicator of phosphorus stress in crops.