Spread ofHeterobasidion annosum(Fr.) Bref. was investigated in a 60-year-old Norway spruce (Picea abies(L.) Karst.) stand in central Lithuania. Of the 375 trees sampled,H. annosumwas isolated from 85 (22.7%). Forty nine (56.3%) of the isolates belonged to the S group and 38 (43.7%) to the P group. Among the S-group isolates, 35 genets occupied 49 trees (1.4 trees per genet on average). In the P group, 24 genets were detected in 37 trees (1.5 trees per genet on average). The largest S and P genets were 21 and 20 m in diameter, respectively. The S group spread was more extensive within stems than for the P group; mean length of the decay column for S-group isolates was 459 ± 159 cm (mean ± SD), and it was 327 ± 101 cm for P-group isolates (p= 0.0001). Average stump area affected by decay was 56 ± 15%for the S group, and 47 ± 16% for the P group (p= 0.016). This is the first published field data on intersterility group related difference in decay extension in tree stems. The S group is better adapted than the P group to growth in Norway spruce