ABSTRACTThree intersterility groups, as well as several stocks which were intersterile with all other stocks, were detected among isolates obtained from North American collections referable to theLaccaria laccatacomplex. Members of these intersterility groups were intersterile with both of the intersterility groups previously identified among tested Swedish isolates referable toL. laccata sensu lato.Genetic divergence was detected between each of the four intersterility groups included in analyses of restriction fragment length polymorphisms of mitochondrial and nuclear ribosomal DNA. Multivariate morphometric analyses, however, only revealed a close correlation between basidioma morphology and intercollection pairing reactions in some cases. The following taxa are recognized from North America based on these studies:L. laccatavar.laccata, L. laccatavar.pallidifolia, L. longipes, L. montana, L. ohiensis(=L. tetraspora) andL. striatula.The tested Swedish material is referable toL. laccatavar.moelleriand, putatively,L. laccatavar.pallidifoliaandL. ohiensis.These studies demonstrate the importance of using a variety of character sets for investigating taxonomic and biological problems in agarics that form ectomycorrhizae.