The midgut of Tenebrio larvae, which reveals a strong reaction for F-actin beneath the apical microvilli after rhodamine-phalloidin treatment, was studied to examine localization of actin. Freeze-fracture replicas of the lateral midgut borders reveal that smooth septate junctions with their characteristic rows of aligned intramembranous particles (IMPs) are found on the upper third of these borders. Thin sections show that short punctate adhering junctions may also occur on this part of the border. Deep etching reveals that the rows of septate junctional IMPs are closely juxtaposed to cytoplasmic fibrils that demonstrate the structural features typical of actin as well as heavy meromyosin labeling. These actin fibrils appear to insert into the junctional membranes. Hence cytoskeletal elements have an intimate spatial association with the membrane modifications typical of intercellular septate junctions and may be involved in the positioning of their component IMPs and also possibly of their septal ribbons.