The vegetative (active, growing, feeding) phase consists of a multinucleate amoeboid mass or sheet (plasmodium). This gives rise to fruiting structures (sporangia) with one to many spores at the head of a stalk. In nearly all species, spores are borne within the sporangium. In Ceratiomyxa, spores are apparently borne externally; each, however, may be a much-reduced sporangium with a true spore within.

Upon germination, a spore releases one or more individual cells known as myxamoebas, which may transform into so-called swarm cells with two flagella (whiplike structures used in swimming). The swarm cells often revert to the amoeboid stage. Formerly, it was believed that reproduction involved the nonsexual fusion of swarm cells, but the process is now thought to be sexual.

The plasmodium, with cytoplasm streaming through it, changes shape as it crawls over or within damp wood, leaves, or soil, ingesting bacteria, molds, and fungi. Characteristically, the entire plasmodium is covered by a layer of slime, which is continually secreted and, as the plasmodium creeps, continually left behind as a network of collapsed tubules. Plasmodia are frequently yellow and orange, but they may also be colourless, red, white, buff, maroon, or, rarely, blue, black, or green.