Fruiting body a plasmodiocarp (sometimes with sessile pulvinate sporangia also present), gregarious to crowded, more or less terete, 0.3–1.0 mm wide and 0.3–0.5 mm high. Hypothallus thin, translucent, colourless to light yellow. Peridium consisting of two layers, the outer layer widely separated from the inner layer, the outer layer composed of a thick, white, smooth, brittle crust of granular lime, the inner layer thin, translucent, membranous, lightly powdered with white lime. Capillitium dense, consisting of nodes that can range from large and angular to branching to small and fusiform, shiny, yellow, ivory or pale cream to white, connected by hyaline threads. Columella a central white or yellow ridge along the length of the fruiting body or consisting only of a thickened base. Spores black in mass, dark purple-brown by transmitted light, evenly and coarsely warted, 11–13 µm in diameter. Plasmodium pale yellow.

Various types of plant debris or (more rarely) living plants, usually near the edges of melting snowbanks in alpine regions.

Mitchel et al. (1986), Neubert et al. (1995).

Physarum alpinum and P. alpestre are very similar morphologically and have not always been recognised as separate species. However, the fruiting body produced by the former is usually a sporangium, while that produced by the latter is usually a plasmodiocarp. Physarum alpestre also has a smoother peridium, a columella, and somewhat larger spores with more prominent warts (Mitchel et al. 1986).[