The portion of the Castilleja secondary haustorium external to host tissues was investigated at the light- and electron-microscopic levels. One or more haustoria may be present along a single lateral root; however, primary roots of Castilleja may also have haustoria. A localized development of root hairs usually precedes haustorial formation. The first sign of haustorial formation is the swelling of root cortical cells which gives rise to a nearly spherical structure. Eventually, the haustorial epidermis is obliterated at the contact surface between parasite and host. The upper haustorium has a complex internal structure consisting of several distinct cell zones. The pericycle gives rise to a parenchymatous zone of cells adjacent to the plate xylem. Later, collenchyma differentiates in the center or core of the haustorium and is surrounded by a cortical zone of parenchyma cells. This is the first report of collenchyma tissue in a haustorium of any parasite. Thus, a mature Castilleja haustorium consists of a plate xylem zone, an adjacent parenchymatous zone, a centrally located collenchyma zone, and a peripheral cortical zone. The endophyte is initiated from hypodermal parenchymatous cells located between the collenchyma core and the haustorial surface. Lipid is abundant in the young endophyte and paramural bodies are common in many cells. Strands of vessel members differentiate at random and are sheathed by thin-walled densely cytoplasmic cells. Vessel members occur within the collenchyma zone as well, and frequently contain starch and other coarsely granular materials. No phloem was found in the upper haustorium of Castilleja. The upper haustorium of Castilleja and of other members of the Scrophulariaceae are compared.