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Terpene content of two distinct collections of the nudibranch <i>Phyllidia coelestis</i> from the South China Sea has been chemically analyzed. A series of amphilectene diterpenes, most likely of dietary origin, with isocyano and formamido functionalities have been isolated from both collections and spectroscopically characterized by an exhaustive nuclear magnetic resonance (NMR) analysis. Interestingly, the structural architecture of compounds <b>5</b>&#8722;<b>7</b> and <b>9</b> with both 8,13-<i>cis</i> and 12,13-<i>cis</i> ring junctions is unprecedented in the amphilectene skeleton. Metabolite <b>3</b>, which was the most abundant in the nudibranch&#8217;s mantle<b>,</b> has been shown to deter feeding by a generalist predator, supporting its involvement in chemical defense.