The distributions of mating alleles in local populations of the litter-decomposing agarics Marasmiellus praeacutus and Collybia subnuda were determined by mating crosses. The tetrapolar M. praeacutus has an unexpectedly low mating allele diversity at both the local and the regional level. This is probably due to a combination of factors which results in limited spore dispersal. The pattern of mating allele distribution among closely adjacent genets suggests that di-mon crossing (the Buller phenomenon) may contribute to the population structure of this species. Collybia subnuda has a mating allele diversity estimated at 45 for the species, with a 95% confidence interval of 19 to 187. On a local scale, closely adjacent genets of C. subnuda did not share mating alleles, indicating that C. subnuda is an outcrossing species. Two partially intersterile groups were identified within the C. subnuda morphospecies. They were not differentiated morphologically, geographically, or by the DNA sequence of the intergenic transcribed spacer (ITS) region of the ribosomal RNA-encoding gene family. Intersterility group (ISG) 1 is usually found on oak leaf litter, and ISG 2 is usually found associated with oak wood. Collection records, mating crosses, and spore-catching experiments indicate that the two ISGs are distributed sympatrically throughout the sampled range. Both ISGs produce binucleate spores in low frequency, and thus have the potential for secondary homothallism. Spore-catching experiments indicate that the spore rain of C. subnuda varies greatly over space and time. Spore viability studies show that C. subnuda spores have a limited viability. The implications of these observations for the population structure and speciation in C. subnuda are discussed. / Ph. D.