Plant size varies by two orders of maginitude within populations of Cucurbita foetidissima HBK. a perennial, prostrate vine of deserts and grasslands in the western USA. Differences in plant size in in this species growing in Kansas (USA) brome grassland were most evident in leaf size (10 times greater in larger vs 13 g in small plants). and taproot mass (c 7000g in large vs 13g in small plants). This study examined the influence of plant size on net photosynthetic CO2 uptake (A), stomatal conductance to water vapour (g), transpiration rate (E), xylem pressure potential and leaf temperature (Tleaf). During a period of drought in which co-occurring grasses and forbs experienced significant water stress (xylem pressure potential< -2*5 MPa), xylem pressure potential in large- and intermediate-sized plants of C. foetidissima was always> -0*6 MPa. In small plants, minimum xylem pressur potential was c-1*3 MPa. No substantial diurnal reductions in A or g were measured in the two groups of larger plants, but CO2 uptake was reduced to zero after midday in small plants, presumably due to water stress. Increased mass of succulent taproots and greater rooting depth may have contributed to greater xylem pressure potential in larger vs small individuals of C. foetidissima. Water use efficiency (WUE) was greater in larger vs small individuals of C. foetidissima. Water use efficiency (WUE) was greater in leaves from large- and intermediate-sized plants relative to small plants under all environmental conditions. During brief periods of low irradiance due to clouds or artificial shading, 5-10oC reduction in Tleaf in plants with large leaves resulted in low E, enhancing WUE. Much smaller reductions in Tleaf and E occurred during shade in the small leaves of small individuals. These results indicate the greater size in C. foetidissima, while not increasing interception of irradiance in this grassland, resulted in greater independence from resource limitations as well as water conservation during periods of variable sunlight. Key words: Cucurbita, photosynthesis, plant size, water stress