The marine derived fungus Alternaria alternata (Fr.) Keissl. was isolated from branch samples of Beibu Gulf, Qinzhou, Guangxi, China and identified by 16s rDNA. Chemical study of A. alternata led to the isolation of a new isomer compound named tricycloalternarene 18c and nine known compounds, including alternariol monomethyl ether, mannitol, cyclo(Gly-Ala), tenuazonic acid, allantoin, thymine, uracil, erythritol, and ergosterol. Their chemical structures were identified by extensive spectrum (IR, MS, 1D, and 2D NMR spectra) and compared with the reported literature data. [ABSTRACT FROM AUTHOR]