Leaf initiation and development, associated with head formation, were monitored during the growth of crisphead and butterhead lettuce plants. The ratio of leaf length to breadth for individual leaves decreased with time and eventually became constant. The ratio, when compared at the same leaf position, was smaller in crisphead lettuce than it was in butterhead lettuce. Two phases were distinguished in the relationship between the rate of leaf primordium initiation and the rate of leaf (longer than 1 cm) production ; the phase transition was earlier in crisphead lettuce than it was in butterhead lettuce. Bags were placed over the younger leaves of butterhead lettuce plants when leaf 15 became longer than 1cm (15th-leaf stage), the dark-grown leaves curled inwards and became narrow. On the 26th-leaf stage, shading did not affect leaf breadth ; and on leaves at positions 25 and higher, the basal parts of the midribs became bent in both shaded and control plants. The results suggest that the sequence of three processes, i.e., changes in the relationship between the rate of leaf primordium initiation and that of leaf production, changes in leaf shape, and inward leaf rolling, play a key role in head formation ; these processes occur earlier in crisphead lettuce than they do in butterhead lettuce.