SUMMARYLolium perenneselection lines with high calculated stomatal resistances to diffusion (rs) as a result of either few or short stomata, maintained leaf extension rates and photosynthetic rates longer than selection lines with low resistances when deprived of water. There were no significant differences between high and lowrsplants in light saturated CO2uptake of turgid attached leaves.When grown in soil drying to 21% moisture, plants with lowcalculated rsexhibited lower minimum leaf resistances (rl) than those with high, measured with a diffusion porometer, on all except the last day. The daily maximumrl(1.5 h after the start of the light period) became greater among low than highrsplants as the difference in rate of leaf extension between the two groups of plants increased with drying soil. Rate of leaf extension was negatively correlated with daily maximumrland started to decline when relative leaf water content (RLWC), at 5 h after the start of the light period, fell below about 88%. Transpiration rate of plants grown in different soil moisture regimes was correlated (r=+ 0.83,P<0.01) with mean maximum adaxial leaf conductance (reciprocal of resistance). There was a highly significant correlation (r=+ 0.62,P<0.01) betweencalculatedadaxialrsand mean minimum measuredrlamong plants growing at high or intermediate soil moisture, but not at low. Therefore, some random variation in minimumrl, even with adequate moisture, seemed to be unaccounted for by variation in stomatal numbers or size. Selection for increased numbers of adaxial stomata also resulted in more on the abaxial surface, but mean adaxial/abaxial ratio in the ‘frequent’ stomata plants was still only about