Variation of caryopsis size and shape in 272 strains of foxtail millet, Setaria italica (L.) P. BEAUV. was studied to clarify the genetic diversity and characterize landraces collected from different regions. A significant difference was detected in the caryopsis size between the strains originally collected from an area extending from the Far East to Southeast Asia and those from other regions. The strains from Japan, Taiwan, Southeast Asia and limited parts of South Asia (Nepal and Bhutan, Orissa, Madhya Pradesh and Maharastra in India) had small caryopses, whereas those from China, most parts of South Asia and its northwestern vicinity, Central Asia and Europe had large caryopses. The strains with round caryopses were mainly found in East Asia, Taiwan Island, some parts of South Asia and western Europe. The center of diversity in caryopsis size and shape was located in South Asia, particularly India and its northwestern vicinity.