The bioactivity-directed isolation of deoxylapachol [I] from a New Zealand brown alga, Landsburgia quercifolia, is described. Compound I was active against P-388 leukemia cells (IC50 0.6 microgm/ml) and was also antifungal. 1,4-Dimethoxy-2-(3-methyl-2-butenyl)-naphthalene [3] was the major low polarity component of extracts of this seaweed, which also contained 2,3-dihydro-2,2-bis(3-methyl-2-butenyl)-1,4-naphthalenedione [6] and 2-(3-methyl-2-butenyl)-2,3-epoxy- 1,4-naphthalenedione 4,4-dimethoxy ketal [7]. Compound 7 was converted to the 2,3-epoxide of I, which had biological activities similar to those of I.