Within-season responses of Pinusresinosa and Pinusbanksiana to fungi vectored by the pine engraver, Ipspini, are strongly influenced by the month of inoculation. The seasonal pattern is a characteristic of the host-fungus interaction, rather than being indicative of either host-or fungal-specific properties alone. The most consistent trend was that early season inoculation resulted in more rapid lesion formation in sapwood than in phloem and that this difference declined as the growing season progressed. Prior inoculation had no effect on necrotic lesion formation. Inoculated trees were examined for three growing seasons. No significant damage related to treatment occurred to either P. resinosa or P. banksiana during this interval. However, Ceratocystisips persisted in living hosts for a longer period than did Ceratocystisnigrocarpa and showed signs of radial and vertical progression throughout the first and beginning of the second growing season. This was especially true in P. resinosa, the host in which the short-term studies revealed the lower lesion growth rate. There was no lesion formation during the third season. Inoculated fungi could be reisolated from within but not outside the resulting lesion.