Iheyamides A (<b>1</b>), B (<b>2</b>), and C (<b>3</b>), new linear peptides, were isolated from a marine <i>Dapis</i> sp. cyanobacterium. Their structures were elucidated by spectroscopic analyses and degradation reactions. Iheyamide A (<b>1</b>) showed moderate antitrypanosomal activities against <i>Trypanosoma brucei rhodesiense</i> and <i>Trypanosoma brucei brucei</i> (IC<sub>50</sub> = 1.5 μM), but the other two analogues, iheyamides B (<b>2</b>) and C (<b>3</b>), did not (IC<sub>50</sub> > 20 μM, respectively). The structure-activity relationship clarified that an isopropyl-<i>O</i>-Me-pyrrolinone moiety was necessary for the antitrypanosomal activity. Furthermore, the cytotoxicity of <b>1</b> against normal human cells, WI-38, was 10 times weaker than its antitrypanosomal activity (IC<sub>50</sub> = 18 μM).