Impermeability to OSO4 of periderm-free zones around the feeding sites of balsam woolly aphid (BWA), Adelges piceae, in the bark of Abies, was caused by a non-suberized impervious tissue (NIT). NIT developed at mechanical injuries in living bark of Abies grandis, Abies amabilis, Tsuga heterophylla, and Thuja plicata in 3 to 4 weeks during summer. It was found at healed injuries regardless of the cause, biotic or abiotic, at abscission scars, and at old resin blisters and rhytidome. It is concluded that NIT precedes formation of necrophylactic periderms (NP), may provide environment necessary for NP formation in tissues internally abutting NIT, and is a marker for distinguishing NP from exophylactic periderms. The imperviousness of NIT to water resulted neither from suberin (seven tests) nor from callose (three tests), and was detected by an F–F test, which is based on penetration of 2% FeCl3 followed by 4% K3Fe(CN)6 through the bark for 3 days each. The formation of NIT in the absence, e.g. rhytidome, as well as in the presence of injuries, BWA, and disease agents, suggests that NIT production is not only a non-specific inherent process but may be the physiological basis of host response to diseases in bark. Pathogen-free, "non-induced" NIT formation at rhytidome as a control process for isolating the host component of host–pathogen interactions is discussed.