Cancer cell line bioassay-guided separation of an extract from a Yukon Territory fungus resulted in the isolation of a new C(16)-terpene dilactone designated yukonin (1) accompanied by two previously known, structurally related constituents (2 and 3). The structure of each was determined by X-ray crystallographic analysis. Dilactone 2 was found to correspond to LL-Z1271alpha isolated from fungi in the genera Oidiodendron, Acrostalagmus, and Holwaya, while dilactone 3 had earlier been prepared by reduction of an alpha-lactol derivative. Each of the dilactones was found to inhibit growth of human cancer cell lines (pancreas, breast, CNS, lung, colon, and prostate) and some pathogenic fungi.