Lithospermum caroliniensehas a heteromorphic incompatibility system complemented by self‐compatibility via cleistogamy. Plants typically are either solely chasmogamic or solely cleistogamic. The percentage of cleistogamic plants in northern Illinois and Indiana populations varied from zero to 4.7. However, the percentage seed from self‐fertilization varied from zero to 51.5, because cleistogamic plants produced 1.5 times as many ovules as chasmogamic plants, and the former type averaged 3.4 seeds per flower in contrast to 0.23 in chasmogamic flowers. Interpopulation differences in cleistogamy and self‐fertilization were strongly correlated with plant density, an increase in density being accompanied by increases in the aforementioned features. Most populations had an excess of thrums, but there was no relationship between cleistogamic seed production and thrum:pin ratio.