Deep stem inoculations with Heterobasidion annosum (Fr.) Bref. were conducted on 34 clones (five to nine rametes) in a 17-yr-old Norway spruce [ Picea abies (L.) Karst.] field trial in southern Sweden. A hole (one-third of the stem diameter) was made and filled with H. annosum -infected sawdust. Eight months later the trees were felled and the stems were sliced into 5-cm-thick discs, 2 m upwards from the point of inoculum. The discs were examined for the presence of the conidial stage of H. annosum and the length of lesion in the inner bark was measured. Infection success was 98.4%. The linear extent of colonized xylem was great (average 140 cm) and in 32% of the trees fungal extension was longer than the collected log (>2 m). There was a significant variation among clones in fungal extension, with a broad-sense heritability ( H 2 ) of 0.18. A significant positive phenotypic and genotypic correlation between tree size and fungal growth was found. There was no significant genotypic correlation between fungal growth and lesion length in the inner bark.