Growth, water relations, and water economy of Poa pratensis 'Merion' were studied as they were affected by infection by Ustilago striiformis or Urocystis agropyri and/or osmotically-induced water stress. Total and leaf weight of U. striiformis-infected plants growing in nutrient solution were greater than healthy plants, although increases were lost under increasing water stress. Root weight and root-shoot ratios were lower in U. striiformis-infected plants than those in healthy plants over all levels of water stress. Total, leaf, and root weights, and root-shoot ratios of U. agropyri-infected plants grown in nutrient solution decreased compared to healthy plants, and the decreases remained as water stress was increased. Infection by U. striiformis had little effect on rhizome growth, but increased tiller growth of P. pratensis. Infection by U. agropyri sharply decreased rhizome growth and greatly increased tiller growth. Tiller and rhizome growth were sensitive indicators of water stress;Infection by either pathogen decreased leaf turgor and water potentials. Healthy plants maintained higher leaf turgor and water potentials than infected plants as nutrient solution osmotic potentials were lowered with polyethylene glycol. Water stress lowered the osmotic potential and relative water content at zero turgor and decreased the turgid weight/dry weight ratios of healthy and infected leaves. Healthy leaves exhibited lower osmotic potentials at zero turgor and smaller turgid weight/dry weight ratios after water stress than infected leaves indicating infection by either pathogen inhibited osmotic adjustment and an increase in cell wall thickness in response to water stress;U. striiformis-infected plants with little or no visible sporulation and healthy plants did not differ in rate of water use or water use efficiency. Infected plants with moderate to heavy sporulation exhibited sharp decreases in water use efficiency and water use compared to healthy plants or infected plants with non-sporulating sori. Stomatal closure on infected non-sporulating sori occurred at lower leaf water potentials than on healthy leaves providing additional evidence for osmotic adjustment in response to pathogen-induced water stress.