The first detailed structural description of the sporophyte of a Neotropical hornwort is presented herein. The morphology and ultrastructure of the placenta, basal meristem, and aerial sporophyte are described in populations of Phaeomegaceros fimbriatus, a poorly known but widespread species, from Panama, Venezuela, Costa Rica, and Ecuador. Gametophyte cells with wall ingrowths and haustorial sporophyte cells lacking ingrowths are closely intermingled in the placenta. Through anticlinal divisions, the basal meristem generates a protoderm, several rows of assimilative cells, and a single ring of archesporial cells surrounding a columella of 16 cells. Further periclinal division in the archesporium produces two or three rows of spore mother cells intermixed with elater initials, both of which are separated and suspended in mucilage as the sporophyte elongates. Spores are finely vermiculate with seven to 10 dimples on the distal surface. Spore walls comprise a perine‐like outer layer derived from the spore m...