The effect of variation in climatic factors has been examined in Mediterranean populations of Phalaris tuberosa covering the climatic range from Morocco and Israel (with severe summer drought and a short growing season of 5-6 months), to northern Italy and northern Greece (with appreciable summer rainfall and a potential growing season of 9-10 months). The populations differed little in germination characteristics at low temperature (4-5[degree] C) but there were large differences in survival at high temperature. The cold requirement for floral induction was closely related to the winter temperature of the place of origin. The optimum temperature for expansion of the leaf surface in all populations was about 24oC day/19oC night but leaf length and leaf width responded differently to temperature changes. The rate of leaf expansion at low temperature relative to that at 24O/19[degree] C was markedly greater in the collections from the southern regions of the Mediterranean, with higher winter temperatures (10-12[degree] C), than in those from the northern regions with winter temperatures of 3-7[degree]C. This variation was based largely on differences in leaf length. The relative growth rate in the early seedling stage was highly correlated with the rate of leaf expansion and was greatest in the Moroccan forms with a short growing season. These differences in relative growth rate were maintained in older plants at the lower temperatures, but disappeared at the higher temperatures as mutual shading developed.