Facultative and obligative marine microscopic fungi are known to be promising sources of various types of biologically active compounds [1,2]. Under the auspices of a program for discovery of biologically active compounds in extracts of isolates of marine microscopic fungi, we isolated the strain Penicillium citrinum Thom. from sediment collected during core drilling of methane gas hydrates at a depth of 645 m in the Okhotsk Sea. The fungus was cultivated for 21 d at 22°C in a 1-L flask containing medium of composition sodium tartrate, 0.005 g; yeast extract, 0.01 g; rice, 10 g; KH 2 PO 4 , 0.005 g; and seawater, 20 mL.Fungal mycelium with medium was extracted twice with EtOAc. The extract was evaporated. The solid was dissolved in EtOH:H 2 O (1:4). The resulting solution was extracted successively by hexane, EtOAc, and BuOH. The EtOAc extract was evaporated in vacuo. The dry solid (300 mg) was chromatographed over a column (25 u 2 cm) of silica gel with elution by hexane:EtOAc (95:5) to afford a compound (8 mg). The compound was identified by MS and 2D NMR spectroscopy (Table 1) and comparison with the literature [3] as (3S)-3,5-dimethyl-8-methoxy-3,4-dihydro-1H-isochromen-6-ol (1). Compound 1, mp 122-123°C (hexane:EtOAc), [D] D 20 +118° (c 0.05, MeOH). Mass spectrum (m/z, %): 208 (100) [M] + , 193 (30) [M -CH 3 ] + , 177 (13) [M -CH 3 O] + , 164 (77), 149 (14), 134 (19), 119 (6), 91 (15), 77 (8).Compound 1 was isolated previously from the terrestrial fungi Penicillium expansum (grain) [4], P. sp. (freshwater isolate) [3], and Penicillium hybrid strains [5]. It is known to exhibit cytotoxic and anticoccidial activity [3]. It was isolated for the first time from a marine fungus.