Effususol A (1), a new 9,10-dihydrophenanthrene, has been isolated from the medullae of Juncus effusus along with ten known compounds, effusol (2), dehydroeffusol (3), juncusol (4), dehydrojuncusol (5), juncuenin B (6), dehydrojuncuenin B (7), juncuenin D (8), luteolin (9), luteolin 5-methyl ether (10), and 4-hydroxy-2,3-dimethyl-2-nonen-4-olide (11). The structure of 1 was elucidated on the basis of spectroscopic data. 2, 4, 6, 7, and 8 have induced caspase-3-mediated cytotoxicity in HT22 cells.