Isozyme variation and somatic incompatibility were investigated in order to study intraspecific genetic variation and to identify genets in natural populations of the ectomycorrhizal fungus Suillus collinitus (Fr.) O. Kuntze, Forty-three isolates, obtained from basidiocarps collected in 11 forest sites under Pinus trees, were examined. Isozyme analysis indicated high intraspecific variation between isolates from the same location and from different locations. Phenetic analysis based on calculated dissimilarity values derived from isozyme banding patterns separated the isolates into two main groups (I and II) with 35% dissimilarity. A specific and highly active acid phosphatase isoform (ACP-2) was detected in most group II isolates. Dikaryotic isolates displaying isozyme similarity coefficient of 100 % were found to be somatically compatible and were considered to originate from the same genet. The same genet was never found at more than two forest sites. This study showed a high correlation between the results of electrophoretic isozyme analysis and those of somatic incompatibility reactions. The results were discussed in terms of population biology.