A comparison was made between the physiological ecology of Daucus carota subsp. carota and D. carota subsp. gummifer, the first a widespread weed, the latter restricted to exposed coastal cliffs in the south-west. Experiments were carried out on germination, growth and flowering both in the field and in the laboratory. These included studies on reciprocal transplants in the field, growth on different soils, culture experiments on the effect of salinity, pH and nutrient concentration. Experiments on the effect of temperature, shade and wind were also conducted. Germination in the field was found to take place mainly in the spring for subsp. carota as the seeds need to be after-ripened over winter, whereas there is no requirement for after-ripening in most of the seeds of subsp. gummifer, so that a large percentage of them can germinate immediately on maturation in the autumn. The optimum temperature for germination of subsp. carota is 20°C, but for subsp. gummifer it is between 200 and 25°C. Weak acids inhibited the germination of both subspecies, whereas alkaline solutions and KNO3 were stimulators, and NaCl stimulated the germination of subsp. gummifer in the dark. Transplanted D. carota subsp. carota grew better inland than subsp. gummifer, whereas transplanted subsp. gummifer grew better at an exposed site at Portland Bill. Wind at Portland Bill broke and killed plants of subsp. carota, whereas those of subsp. gummifer survived and were undamaged. Sodium chloride had a beneficial effect on the growth of subsp. gummifer even at a concentration of 1 g/l, which inhibited the growth of subsp. carota. The highest wind speed produced in a wind tunnel (3 m/sec), decreased the growth of subsp. carota, whereas growth of subsp. gummifer increased with increasing wind speed. The main conclusion of this study is that air temperature, wind speed and the salinity of the soil are the most important factors determining the distribution of the two subspecies.