<jats:p> Two unusual compounds, which are made up of a sterol portion fused at C-5 and C-6 to an atrovenetin-like part, have been isolated from a fungus that causes Sirococcus shoot blight of spruce. The structures of these compounds, sirosterol (4) and dehydroazasirosterol (13), were determined by spectroscopic methods and comparison with appropriate model compounds. Sirosterol is an adduct of ergosterol and atrovenetin (5) and dehydroazasirosterol is an adduct of 9(11)-dehydroergosterol and an azaatrovenetin. Both compounds give a monomethyl ether when treated with diazomethane and both have been transformed to the corresponding 3-ketosteroids. The preparation of the triol 6 resulting from the selective hydroxylation of the 5,6-double bond in ergosterol is described. A possible biogenesis of sirosterol from ergosterol endoperoxide and a desoxyatrovenetin is discussed.