A new succulenticolous Myxomycete species, Didymium wildpretii, found on decaying remains of various species of cacti, is described from two arid zones of the world. This species was collected from central Mexico, at the southern limit of the Chihuahuan Desert, and from the Canary Islands (Spain). The new species has small, pale yellow sporocarps, 0.1-0.7 mm high, that are sessile or have short, orange-yellow, calcareous stalks and small, uniformly warted spores. The stability of the taxonomic characters of the species was confirmed with both moist chamber cultures and spore-to-spore culture on agar. Life cycle events are described from germination to sporulation. Myxomycete specimens were examined with scanning electron microscopy and light microscopy, and micrographs of relevant morphological details are included.