Within populations of Trifolium repens L. and Festuca rubra L. growing on the sand dune system at Aberffraw, Anglesey, Triffoolium repens shows ecotypes differentiated by their response to soil water status, but Festuca rubra does not. All populations of F. rubra grew best with moderate, and were adversely affected by both high and low, soil moisture status. Populations of T. repens showed a site-specific growth response; plants from 'dry' habitats were least affected by low soil moisture levels, whereas those from 'wet' habitats were quite adversely affected by low soil moisture status. Such differential response suggested the possible existence of these populations as ecotypes. More evidence was supplied by the reciprocal growth of plants from wet and dry sites on both sites in the field; each performed better when grown on its original site. Physiological differences between ecotypes of T. repens at -1.0 MPa in the rooting medium, provided by solutions of polyethylene glycol (m. w. 4000), were investigated. Plants from the wet site were not able to withstand such low water potential, and steadily their pressure potentials decreased, they lost turgor and wilted. Plants from the dry site showed their ability to keep their pressure potentials constant and thus maintain turgor, as their water potentials dropped. This ability of turgor maintenance was shown to be accomplished by osmotic adjustment through solute accumulation. Plants mainly accumulated K+, Na+, and the sugars sucrose, glucose, and fructose. However, low osmotic potentials in plants from the wet site were only due to tissue dehydration and consequent concentration of solutes in the cells. Possibly as a consequence of turgor maintenance in plants from the dry site, their stomatal resistance did not increase substantially as did that of plants from the wet site at low water potential, and therefore they were able to maintain relatively higher rates of net photosynthesis.