Abstract: The occurrence of myxomycetes was studied on fallen trunks of Picea abies, Pinus sylvestris, and Betula spp. (B. pendula and B. pubescens) in a small oldgrowth forest in southern Finland in May 1998 September 1999. The study site is located in Luukkaa Recreation Area, and was left in pristine state in 1966. The sample trunks were chosen to represent different stages of decomposition and checked every second or fourth week in all for 17 times. A total of 325 myxomycete specimens representing 44 taxa in 16 genera were observed. Four taxa, Comatricha pulchella var.fusca, Lycogala exiguum, Licea cf. pusilla, and Physarum bethelii are new records to Finland. During the study, myxomycetes were most abundant on decomposing trunks of Betula spp. (123 specimens), especially on well-decayed trunks. The species diversity was about the same on all studied tree species: 27 taxa were recorded on P. abies and P. sylvestris, and 22 on Betula spp. The largest diversity was on two pine trunks with fairly soft wood (a total of 19 taxa). The common myxomycetes, Ceratiomyxa fruticulosa and Lycogala epidendrum, represent generalists in this study, being abundant and present at nearly all decay stages of all studied tree species. Leocarpus fragilis and Tubifera ferruginosa are examples of species that preferred well-decayed wood, and Stemonitopsis reticulata and S. typhina were found only on hard to fairly hard wood. The peak of the number of taxa and abundance of myxomycetes occurred from July through to September.