We transformed tobacco plants (Nicotiana tabacum L., Xanthi) by introducing a sense construct of NtFtsZ1-2. This tobacco nuclear gene encodes a chloroplast-localized homologue of FtsZ, the bacterial cell-division protein. The over-expressing plants contained enlarged chloroplasts in their leaf mesophyll cells. In the T$_1$ progeny, we observed three different phenotypes: 1) plants with cells containing many small chloroplasts, which was the same as for wild-type plants; 2) plants in which the cells contained one to three enlarged chloroplasts (severe type); and 3) plants whose cells contained a combination of many small chloroplasts and one to three enlarged chloroplasts (intermediate type). The outward appearance of the severe and intermediate types of transgenic plants did not differ noticeably from the wild-types. However, the severe-type plants were most retarded in their growth under both high- and low-light conditions, followed by the intermediate-types. Under medium levels of light, the two types of transgenic plants exhibited growth rates comparable to that of the wild types. Based on the overall results, we suggest that many small chloroplasts, rather than a few large chloroplasts, are required for efficient use of light energy in the mesophyll cells.