Rooted micropropagated plantlets of Aronia arbutifolia were transplanted into sterile rootcubes and placed into a growth room under three irradiance levels. The irradiance levels consisted of 350 (low), 550 (medium) and 700 (high) pmol m~2 s . Plants were harvested from day one and at 10-day intervals. Shoot growth, number of leaves and leaf area were significantly affected by irradiance treatments. Plants grown at the lowest irradiance level developed longer shoots, larger leaf area and more roots. Repeated measurements analysis on shoot length determined that the response to low irradiance was significantly greater from the other two treatments. Seventy percent of the plants in the highest irradiance treatment died before 3 0 days. Photosynthetic rate was greater in plants grown under high irradiance. Transmission electron micrographs were taken to determine chloroplast development, including grana stacking under the three irradiance levels.