Artificial inoculations in the Cupressaceae proved that <I>Seiridium cardinale</I> was more virulent than <I>S.cupressi</I> and <I>S.unicorne. Cupressus macrocarpa</I> was found to be highly susceptible to <I>Seiridium</I> canker, <I>C.sempervirens</I> very susceptible, while <I>C.torulosa</I> and <I>C.arizonica</I> were moderately susceptible. <I>Chamaecyparis lawsoniana</I> was highly resistant to <I>S.cardinale</I>, but very susceptible to <I>S.unicorne</I> and moderately susceptible to <I>S.cupressi</I>. Intraspecific variation in susceptibility to <I>S.cardinale</I> was found in <I>C.sempervirens. S.cupressi</I> was more pathogenic than <I>S.unicorne</I> on <I>C.macrocarpa, C.arizonica</I> and <I>C.torulosa</I>, whereas it was less pathogenic on <I>C.sempervirens</I>. Mature bark proved to be more resistant to <I>Seiridium</I> canker than young bark. Low variability in pathogenicity of <I>S.cardinale</I> was found, with only one isolate out of eight proving to be a weaker pathogen. Histological examination of bark of cypress seedlings following infection with <I>S.cardinale</I> revealed the formation of strong necrophylactic periderm as an important resistance mechanism against <I>Seiridium</I> attack, and was particularly marked in <I>C.lawsoniana</I>. Strong necrophylactic periderms were detected in resistant and tolerant clones of <I>C.sempervirens</I>, whereas weak or a series of easily re-invaded ones were found in susceptible clones. Variations in pathogenicity of <I>Seiridium</I> in <I>in vitro</I> inoculations of micropropagated cypress shoots paralleled results found in the glasshouse. Wounding of micropropagated shoots significantly increased the size of lesions caused by all three <I>Seiridium</I> spp. In axenic conditions, hyphae of <I>Seiridium</I> spp. penetrated host tissues through stomatal apertures or directly through the cuticle. Under these conditions, infected tissues of <I>C.lawsoniana</I> formed ligno-suberized barriers as a result of fungal invasion, whereas those of <I>C.sempervirens</I> did not.