The structures of haliclonacyclamines A (1) and B (2), and their methiodide salts (3) and (4), were investigated by 1D- and 2D-NMR experiments, notably DQFCOSY, HMBC, HMQC-HOHAHA, and HOHAHA. The relative stereochemistry and position of alkene substituents were determined by single crystal x-ray study at low temperature. The parent haliclonacyclamines show pronounced cytotoxic, antibacterial and antifungal activity. Copyright (C) 1996 Elsevier Science Ltd