Efrapeptins, a group of peptide toxins with mitochondrial ATPase inhibitory and insecticidal properties isolated from the culture filtrates of the fungus Tolypocladium niveum, had unknown complete structures despite a prior partial sequence report. This study establishes the complete stereostructures of five efrapeptins (C-G), including their amino acid sequences and evidence for the C-terminal blocking group—a bicyclic amine that plays an important role in biological activity. These peptides are composed of 15 amino acid residues, are rich in α-aminoisobutyric acid (Aib), have an acetylated N-terminus, and are distinct from fungal peptaibols (which have an amino alcohol at the C-terminus).