The spores of myxomycetes are nearly all spherical with a diameter from 4 to 20 μm (micrometers). Some are decorated with spines, warts or reticulations, but in many species these important identifying features are indistinct and difficult to see even with a good compound microscope. In some cases (especially in the Stemonitales) it is necessary to look at spore ornamentation with a Scanning Electron Microscope (SEM).
Most myxomycete spores are hydrophobic (i.e. resistant to wetting) and will float on water. It is possible that lipid (which has been found in Fuligo septica) causes the floatation rather than the pattern of rodlets on their surface as is the case for fungi. (Lipids are small organic molecules that are only sparingly soluble in water but soluble in organic solvents.)
The cell walls contain melanin, a dark pigment that in fungi is known to protect against damaging radiation and lysis by soil micro-organisms; it probably fills the same role in myxomycetes. (Lysis is the dissolution or destruction of cells by an action that disrupts the cell membrane.)
Spores are either light, rusty or dark brown, purple black, black, white, pink, yellow, orange or red.
Spores are generally believed to be dispersed by wind and air currents but the numerous invertebrates (especially beetles) associated with fruiting bodies also play a role in their dispersal.
Spores are free in most species but are clustered in Dianema corticatum, Minakatella longifila and several Badhamia species.
If spores in one fruiting body vary greatly in size it usually indicates abnormal development