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A new aromatic bisabolene-type sesquiterpenoid (7 S ,8 S )-8-hydroxysydowic acid ( 1 ), together with a pair of new bisabolene derivative (±)-(7 R* ,10 R* )-10-hydroxysydowic acid ( 2 ), as well as two known analogues hydroxysydonic acid ( 3 ) and sydowic acid ( 4 ) were isolated and identified from the endophytic fungal strain Aspergillus sydowii EN-434 that was obtained from the marine red alga Symphyocladia latiuscula . Their structures were elucidated by nuclear magnetic resonance (NMR), high resolution electrospray ionization mass spectroscopy (HRESIMS), and quantum chemical electronic circular dichroism (ECD) calculations as well as comparison of the data with literature reports. Compound 1 exhibited potent DPPH free radical scavenging activity with IC 50 value of 113.5 μmol/L.