Theriophonum infaustum N.E. Br. (Araceae) is monoecious with unisexual flowers. The female flowers are with unilocular ovaries having basal and apical suborthotropous ovules. The present study of the floral vascular organization revealed that the ovary is tricarpellate syncarpous, but unilocular and hence pseudomonomerous. The basal and apical placentae are truly basal-parietal and apical-parietal, and they originated by the elimination of septa and ovules along the central region. Vascular organization in the female flowers was found to be diverse, having fused and independent lateral bundles of corresponding carpels of the tricarpellate syncarpous unilocular gynoecium. The male flowers were supplied with single unbranched staminal bundles and this is a derived condition from the primitive multiple- traced condition. The neuter flowers were also supplied with single bundles resembling that of staminal bundles. The sterile spadix appendix was presumed to have formed by the fusion of aborted staminate flowers as evidenced by the lateral bundles produced at the lower half of the appendix.