The isolation and structure elucidation of two cyanobacterial debromoaplysiatoxin (DAT) analogues, neo-debromoaplysiatoxin A (1) and neo-debromoaplysiatoxin B (2), were reported and found to possess 6/10/6 and 6/6/6 fused-ring systems, respectively, which are rarely seen among aplysiatoxins. Both compounds exhibited potent blocking activity against Kv1.5 with IC50 values of 6.94 +/- 0.26 and 0.30 +/- 0.05 mu M, respectively. These findings suggest the potential of aplysiatoxin analogues in modulating ionic channels and also provide links between the DAT target, protein kinase C, and cell regulation.