Two experiments were conducted in a glasshouse to evaluate the effect of root rot caused by Pythium irregulare on water use efficiency (WUE), stomatal conductance (g_s), photosynthesis rate (A) and intercellular CO₂ concentration [CO₂]_i of wheat (Triticum aestivum L. cv. Janz), exposed to drought from full anthesis until harvest. In the first trial, four inoculum densities (0.1, 0.5, 2 and 5 g colonized millet seed/pot) of Pythium were compared with a control. The results showed no significant effect on grain yield and WUE calculated on a shoot biomass or grain basis, between infected and control plants. In the second trial, infected plants had significantly higher (P < 0.01) WUE for both shoot biomass and grain than uninfected plants when inoculum density of Pythium was increased up to 10 g/pot, although grain yield was not affected. Predawn leaf water potential (Ψ) was not significantly affected by Pythium during the drought. However, diseased plants had significantly lower midday Ψ at 14 and 21 days after withholding water, when compared with controls. Instantaneous transpiration rate (E), and g_s of controls, were reduced after 7 days of water stress compared with infected plants. This could be because infected plants entered the drought with more available water due to reduced transpiration during early growth. However, A and instantaneous WUE (A/E), were only higher in controls prior to the drought perhaps due to higher activity of Pythium in moist soil. The decrease in A before drought was not due to a decrease in g_s for diseased plants. Pythium had no effect on [CO₂]_i at all measurement times. The insignificant effect of Pythium on photosynthetic rate may provide ideas for improvement of WUE of wheat under post-anthesis drought. The effect of Pythium infection on photosynthetic capacity under these conditions needs further investigation.