The apparent efficiency of conversion of incident light energy (ɛapp/i) by whole plants of Im-patiens parviflora in a range of natural and artificial light climates has been calculated from growth analysis data. It was found that it ranged from 1.3% in full natural daylight to 7.5% in extreme shade (metal shades transmitting 7% daylight). The corresponding figures for real efficiency, i.e. corrected for respiration losses, ɛtrue/i were 3.0% and 12.5% respectively. In the growth cabinets with light intensities roughly equivalent to 7—15% daylight a combination of two Blue/six De Luxe Warm White fluorescent tubes gave values of ɛapp/i averaging 7.4% for plants grown at 15°C, 5.9% for those at 10°C and 6.5% at 20°C. Four Daylight/four De Luxe Warm White averaged 5.8% at 20°C and Red (magnesium arsenate) at 20°C was highest with 7.7%. Blue tubes gave 6.0% apparent efficiency with plants at 15°C. In all the cabinet experiments and in the extreme shade in natural daylight the plants were small with leaves scarcely overlapping. In more open, natural daylight sites some self-shading would have occurred. When the efficiency was expressed on a quantum basis, rather than on total energy, and correction for temperature differences applied, there was little difference between Blue, Blue-white and Red, with Daylight-white somewhat lower. On the basis of assimilation for a given number of lamps the combinations two Blue/six De Luxe Warm White and four Daylight/four De Luxe Warm White were the best.