Solvent extractable organic compounds of fossil conifer cones were analyzed using gas chromatography-mass spectrometry to investigate the preservation of resin compounds in fossil plants. A paleosoil in sand deposits (Pleistocene) near Newport, OR, contains twigs and seed cones of several conifer species. One seed cone, possibly Douglas Fir (Pseudotsuga sp., Pinaceae), was studied. Seed cones of three species of Cupressaceae (Cunninghamia chaneyi, Glyptostrobus oregonensis, Taxodium dubium) from the Miocene Clarkia flora, ID, were analyzed for comparison. Diterpenoids are the major components in the extracts of all samples analyzed, and all species show characteristic terpenoid patterns. Extract of the seed cone from Newport contains a series of degraded abietane type diterpenoids (fichtelite, 19-norabieta-8,11,13-triene, 18-norabieta-4(19),8,11,13-tetraene, 18-norabieta-8,11,13-triene, dehydroabietane, simonellite, 1,2,3,4-tetrahydroretene, retene, norisopimarane, ent-18-norkaurane), with fichtelite as the major compound. These diterpenoids are interpreted as the diagenetic derivatives of natural product precursors such as abietic and pimaric acids. The derivatized extract reflects diagenetic alteration by hydrolytic, oxidative and reductive processes of the cellulose and resin acids of the cone. Dehydroabietol, 3-oxoabietic acid and dehydroabietic acid are oxidized derivatives of the resin acids, and - and -glucose, mannitol, arabitol and cellobiose are the hydrolysis products from cellulose. Phenolic abietanes (ferruginol, 6,7-dehydroferruginol) are the predominant terpenoids in the Clarkia samples. Closely related species Taxodium dubium and Glyptostrobus oregonensis show similar patterns of phenolic abietanes (sugiol, taxodione acetate, 12-hydroxysimonellite), and Cunninghamia chaneyi is characterized by a low ferruginol content and hinokiol and hinokione. The terpenoid compositions of the fossil species resemble those in related extant species. Diterpenoid acids of the abietane and pimarane classes are the major components of the Pinaceae resins while phenolic abietanes (ferruginol and its derivatives) are characteristic for species of the Cupressaceae. Results show that specific terpenoids are preserved in fossil conifers which can be used as their chemosystematic markers.