Four new halogenated chamigrenes have been isolated from the sea hare, the (E)- and (Z)-9-(bromomethylidene)-1,2,5-trimethylspiro[5.5]undeca-1,7-dien-3-ones (11 and 10, resp.) the structures of which were deduced by NMR spectroscopy and by comparison with a known relative, the (8R,9R)-8-bromo-9-chloro-5,5,9-trimethyl-1-methylidenespiro[5,5]undec-3-en-2-one (12), and its (11 R)-11-acetoxy-substituted derivative 13. Their structures and absolute configurations were determined and deduced by X-ray using the absolute-structure parameter. The configurations were remarkable in being enantiomeric to those of the tertiary chlorochamigrenes isolated so far. Consequently, a more general scheme is proposed to account for their biogenesis.