The data obtained by rating 50 plots in red × black spruce populations throughout central New Brunswick with a hybrid index indicate that red and black spruces have hybridized extensively. In the New Brunswick Lowland, gentle slopes and flat uplands form a continuum between characteristic habitats, permitting extensive contact between the two species. Where a considerable zone of contact was present, hybrid populations were established. Selection pressure is apparently strong, for despite the ideal opportunities for contact and the fertility of the hybrids, parental species remain phenotypically pure in their respective characteristic habitats (as far as these could be defined). The composition of hybrid populations was related to the extent of resemblance of 'intermediate' sites to parental site preferences. Introgressed black spruce predominated in hybrid populations, possibly due to the overriding influence of disturbances such as fire, logging, and damage to red spruce types by spruce budworm.