The diversity and distribution of mating alleles in local populations of two litter decomposing mushroom species were investigated. The results from mating crosses between single-spore isolates were used to deduce mating genotypes among closely adjacent individuals.Collybia subnudahas a high mating allele diversity that is comparable to the diversity of other bipolar species. Adjacent individuals ofC.subnudadid not share mating alleles, which suggests that outcrossing usually occurs in this fungus. The estimate of mating allele diversity inC.subnudais 45, with a 95% confidence interval of 19–187. A simplified approach for designing and analyzing intercollection mating crosses for tetrapolar fungi was tested withMarasmiellus praeacutusand found useful. An unexpectedly low mating allele diversity was found inM.praeacutus, with adjacent individuals frequently sharing more than one mating allele. The pattern of shared alleles inM.praeacutussuggests the occurrence of di–mon crossing (the Buller phenomenon). We suggest that the observed low level of mating allele diversity inM.praeacutusis a consequence of limited spore dispersal, and we discuss the implications of these conclusions for population sampling in the higher fungi.Key words:Collybia subnuda,Marasmiellus praeacutus, mating, population.