Deterioration of poplar pulpwood (a mixture of Populus tremuloides Michx. and P. balsamifera L.) on Manitoulin Island, Ontario, was studied in field, pit, and block pile storage with reference to the reaction of the rot fungi present in the living tree to storage conditions, and the penetration of fungi which attack wood subsequent to felling. The rots found most prevalent in the living trees, and the most important from the standpoint of volume of wood affected, were the white trunk rots caused by Fomes igniarius var. populinus (Neu.) Campb., Radulum casearium (Morgan) Lloyd, Polyporus dryophilus var. vulpinus (Fr.) Overh., a rot associated with a non-Basidiomycete designated in this paper as Fungus X, and the brown rot produced by Poria cocos (Schw.) Wolf. The first four rots may survive in storage, but are not appreciably active. Butt rots caused by Armillaria mellea (Vahl.) Quél., Pholiota spectabilis (Fr.) Quél., and a gray–brown butt rot were not uncommon, but of limited importance both on account of the amount of wood attacked and the practical inactivity of the causal agents in storage. The only tree-rot fungus found likely to cause damage in storage was P. cocos; it produces brown rot and is capable of spreading vigorously in stored wood. Other tree rots occurred, but were of minor importance. Stereum purpureum Pers. was the first fungus to infect the wood after the trees were felled; it spread rapidly and fruited during the first summer. It was followed by three species of Polyporus, P. adustus (Willd) Fr., P. hirsutus (Wulf.) Fr., and P. versicolor (L.) Fr. These became established in the wood during the first summer; they fruited on field and pit logs during the second season, and were fruiting luxuriantly on block pile logs which had been stored three years. They produce white rots and in some logs caused advanced decay in two years' storage. Other storage rot and staining fungi were isolated from the bolts under study, but during the period of observation did not effect wide spread penetration. The same pattern of decay and succession of fungi were found in all types of storage investigated.