Plants were established for 17–23 days in lights of 17–19 cal/cm2/day (≡ 10% summer daylight) using 16 hours fluorescent light in controlled environment cabinets and were then transferred to lower light intensities in the range 1.5–8.7 cal/cm2/day (≡ 0.9–5% summer daylight). The unit leaf rate was linearly related to light intensity over the range 0–20 cal/cm2/day and the compensation point was approximately 0.3 cal/cm2/day. The changes in unit leaf rate were closely followed by marked morphogenetic changes, even in plants growing in light intensities so low that there was scarcely any net gain in weight. Over the dry weight range 6–35 mg low light intensity did not evoke a great increase in leaf area ratio as, over this range in size, the principle cause of increased leafiness was the increase in leaf weight ratio, which is scarcely affected by light intensity. Plants of greater than 35 mg dry weight showed the strong reciprocal relationship between unit leaf rate and leaf area ratio due to the effect on specific leaf area. At very low light intensities the increase in leaf area ratio was not adequate to offset the fall in specific leaf area and the relative growth rate was diminished sharply over the light range from 15 cal/cm2/day to the compensation point for light. The significance of these physiological features for the aut-Ecology of Impatiens parviflora is discussed.