Fontonamide (3) and anhydrohapaloxindole A (4) are two new minor alkaloids isolated from a cultured strain of the terrestrial blue-green alga Hapalosiphon fontinalis. Both compounds are singlet oxygen oxidation products of hapalindole A (1), the major antimycotic alkaloid in this cyanophyte. Their structures and relative stereochemistries were elucidated using a combination of spectral techniques, including UV, IR, mass spectrometry (EI, high-resolution), and nuclear magnetic resonance (¹H, ¹³C, DEPT, heteronuclear chemical shift correlation (CSCM), selective INEPT), as well as nuclear Overhauser effect (NOE) studies. Hapalonamide A (5), the probable precursor of fontonamide, is formed alongside 3 and 4 when hapalindole A is subjected to singlet oxygen oxidation in pH 8 buffered aqueous methanol containing rose bengal, though 5 has not been detected in H. fontinalis extracts. Further oxidation studies of hapalindole A in methanol and buffered aqueous methanol yielded additional products, including 10-methoxyhapalindole A (7), hapaloxindole A (8), hapalonamide G (6), and minor hapalindole derivatives (G, I, K), whose structures were also characterized. The absolute stereochemistry of hapalindole A (10R,11R,12R,13R,15S) and related compounds (3, 4, 7, 8, etc.) was inferred from optical rotation data, NOE experiments, and comparison with natural products (e.g., hapalindole K with known absolute stereochemistry via X-ray crystallography).