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Five new bicyclic carboxylic acids were obtained by antibacterial activity-guided isolation from a Korean colonial tunicate <i>Didemnum</i> sp. Their structures were elucidated by the interpretation of NMR, MS and CD spectroscopic data. They all belong to the class of aplidic acids. Three of them were amide derivatives (<b>1</b>–<b>3</b>), and the other two were dicarboxylic derivatives (<b>4</b> and <b>5</b>). The absolute configurations were determined by a bisignate pattern of CD spectroscopy, which revealed that the absolute configurations of amides were opposite to those of dicarboxylates at every stereogenic centers. Compound <b>2</b> exhibited the most potent antibacterial activity (MIC, 2 μg/mL).