Oxazole-containing macrolides (1-5) isolated from the marine sponge Chondrosia corticata were evaluated for their actin depolymerizing activities by monitoring fluorescent intensity of pyrene F-actin. These studies led to the identification of (19Z)-halichondramide (5) as a new actin depolymerizing agent. The actin depolymerizing activity by (19Z)-halichondramide (5) was four times more potent than that of halichondramide (1). Compounds 1 and 5 also have potent antifungal activity. The preliminary structure-activity relationship of these compounds is described to elucidate the essential structural requirements.