Nivicolous myxomycetes occur at the edge of spring-melting snow in mountainous areas. They are mostly considered cosmopolitan species morphologically and ecologically uniform across their entire distribution ranges. Thus, long-distance dispersal has been suggested to be the main mechanism shaping their ranges and geographical variability patterns. To test this hypothesis we conducted the first detailed analysis of morphological variability, occurrence frequency and phenology of nivicolous myxomycetes collected in the hitherto unexplored Austral Andes of South America (southern hemisphere = SH) in the comparative context of data from the northern hemisphere (NH). We used Stemonitales, the most representative and numerous taxonomic order in nivicolous myxomycetes, as a model. A total of 131 South American collections represented 13 species or morphotypes. One of them, Lamproderma andinum, is new to science and described here. Several others, L. aeneum, L. album, L. pulveratum, "Meriderma aff. aggregatum ad. int.", M. carestiae and "M. spinulosporum ad. int.", were previously unknown from the SH. Lamproderma ovoideum is reported for the first time from South America and Collaria nigricapillitia is new for Argentina. The fine-scale morphological analysis of all species from the study area and reference NH material demonstrated a high intraspecific variability in most of them. This suggests isolation and independent evolutionary processes among remote populations. On the other hand, the uniform morphology of a few species indicates that long-distance dispersal is also an effective mechanism, although not as universal as usually assumed, in some nivicolous myxomycetes. Analysis of nivicolous species assemblages also showed significant differences among major geographic regions in that the Stemonitales were significantly less common in the SH than in the NH. Furthermore, the occurrence of nivicolous species in summer and autumn, out of the typical phenological season, is recognized as a possible distinctive phenomenon for the SH populations.