Chloroplast replication in cultured cells of tobacco (Nicotiana tabacum cv. Samsun NN) was investigated by electron microscopy in comparison with that of green leaves. The structure of chloroplasts in cultured cells changed conspicuously during cell growth especially in photoautotrophic cells. The frequency of dumbbell-shaped chloroplasts (intermediate of chloroplast division) was the maximum (about 23% of total chloroplast) in photoautotrophic cells at 3 days after inoculation, before the cells had started to grow. By contrast, in photomixotrophically cultured cells, the highest frequency of dividing chloroplasts was observed at the early exponential phase (about 7 days after inoculation). The dividing chloroplast was hardly detected in green leaves even at a young stage. The advantages of cultured cells for the study of chloroplast replication and ultrastructural development are discussed.