An outbreak of balsam fir dieback in Ontario, which was first reported in 1954, reached its peak throughout the Province in 1955 and gradually receded during subsequent years. The dieback, which involved leaders, branches, and occasionally entire trees, appeared when those parts died as the result of their proximal invasion by a fungus. Cankering usually accompanies necrosis; fruiting bodies may be produced immediately or may not appear until the spring following canker formation. The three fungi most commonly found in association with dieback, Thyronectria balsamea (Cke. & Pk.) Seel., Dermea balsamea (Pk.) Seav., and Valsa abietis Fr., were shown to be independently capable of inducing typical dieback and cankering when inoculated into wounds on transplanted trees. It was further shown that adult balsam fir borer beetles (Monochamus spp.), which regularly feed on and injure twigs and branches of healthy trees, usually carry viable material of one or more of the three common fungi. The ability of the beetles to kill twigs was confirmed and it was shown that, although dieback frequently develops from wounds made by fungus-infested beetles, no dieback ensued in the absence of fungi. The outbreak is believed to have been caused by the interaction of several factors (climate, fungi, site, and insects) on the host trees.