Foxtail millet (Setaria italica (L.) Beauv.) is a soft-stemmed summer cereal crop that is grown at a high crop density; however, stem lodging is recognised as a serious yield-limiting factor. The stem-breaking strength of the third to fifth basal internodes has previously been identified as the most important factor in determining the lodging resistance of foxtail millet. We measured variation in stem-breaking strength, length and weight of these internodes at different developmental stages and in response to different crop densities by using foxtail millet cultivars that differed in lodging resistance. The stem-breaking strength of the third internode was greater than of the fourth and fifth internodes, which had comparable stem-breaking strengths; this result was independent of genotype and developmental stage. The stem-breaking strengths of the three internodes were significantly correlated with each other and linearly related across different developmental stages and crop densities. The stem-breaking strength of the three internodes at hard dough stage (Zadoks growth stage Z87, at 30 days after flowering) was less than at other developmental stages in the lodging-resistant genotypes. Stem-breaking strength for the three internodes was correlated with fresh weight (FW) and dry weight (DW) per cm. The variation of FW and DW per cm of the internodes in response to crop density was attributed to the change of FW and DW of the corresponding internodes, rather than to variation in their length. Results from this study showed that the fourth or fifth basal internode was more prone to lodging than the third basal internode because of their lesser stem-breaking strength. Crop density linearly influenced the stem-breaking strength of the three basal internodes. Reducing crop density enhanced stem-breaking strength of third to fifth internodes, which may reduce the risk of stem lodging in this crop. Based on the findings, the stem-breaking strength of the fourth or fifth basal internode measured at the late grain-filling stage from Z77 (late milk) to Z87 (hard dough) differentiated stem-breaking strength, the most important stem lodging associated parameter, in the foxtail millet genotypes.