Two new modified cyclic hexapeptides, venturamides A (1) and B (2), were isolated from the marine cyanobacterium Oscillatoria sp. by antimalarial bioassay-guided fractionation. The isolation of 1 and 2 represents the first example of the identification of cyanobacterial peptides with selective antimalarial activity. The planar structures of 1 and 2 were determined by 1D and 2D NMR analyses and, in the case of venturamide A (1), comparison with the literature data for a previously reported synthetic compound. The absolute configuration of the amino acid residues was determined by selective hydrolysis in conjunction with Marfey's analysis. Compounds 1 and 2 were tested for biological activity against a range of tropical parasites.