A novel bioactive lipid, cyclic phosphatidic acid (cPA), was isolated originally from myxoamoebae of a true slime mold, Physarum polycephalum, and has now been detected in a wide range of organisms from slime molds to humans. It has a cyclic phosphate at the sn-2 and sn-3 positions of the glycerol carbons, and this structure is absolutely necessary for its activities. This substance shows specific biological functions, including antimitogenic regulation of the cell cycle, regulation of actin stress fiber formation and rearrangement, inhibition of cancer cell invasion and metastasis, regulation of differentiation and viability of neuronal cells, and mobilization of intracellular calcium. Although the structure of cPA is similar to that of lysophosphatidic acid (LPA), its biological activities are apparently distinct from those of LPA. In the present review, we focus mainly on the enzymatic formation of cPA, the antimitogenic regulation of the cell cycle, the inhibition of cancer cell invasion and metastasis, and the neurotrophic effect of cPA.