Terpenoid isonitriles have been isolated from several sponges, mainly those of the order Halichondrida, and often coexist with the corresponding isothiocyanates, formamides, and primary amines. We wish to report the isolation and structural elucidation of two carbonimidic dichlorides from the marine sponge Pseudaxinyssa pitys, the first natural products found to contain this rare functionality. The first carbonimidic dichloride 1 (C₁₆H₂₃NOC₁₄) was obtained by methanol extraction and Florisil chromatography, with its structure confirmed by mass spectrometry, ¹³C NMR, infrared spectroscopy, and reactions (reduction to isonitrile 3, hydrolysis to formamide 4, treatment to primary amine 5 and methyl urethane 6). Hydrogenation, ozonolysis, and Eu(fod)₃-induced shift analysis revealed a new sesquiterpene skeleton axinyssane. A second carbonimidic dichloride 12 (C₁₆H₂₄NC₁₃) was isolated as a minor product, with its functionality confirmed by infrared and ¹H NMR spectra. We propose that the carbonimidic dichlorides may result from enzymatic chlorination of the corresponding isonitriles (not detected), and the axinyssane skeleton may arise from a chloronium ion-initiated cyclization. The isonitriles derived from these carbonimidic dichlorides inhibit the growth of Staphylococcus aureus.