Two new members of the amphidinol family, amphidinol A (1) and its 7-sulfate derivative amphidinol B (2), were isolated from a strain of Amphidinium carterae of Lake Fusaro, near Naples (Italy), and chemically identified by spectroscopic and spectrometric methods. Amphidinol A showed antifungal activity against Candida albicans (MIC = 19 µg/mL). Biosynthetic experiments with stable isotope-labelled acetate allowed defining the elongation process in 1. For the first time the use of glycolate as a starter unit in the polyketide biosynthesis of amphidinol metabolites was unambiguously demonstrated.