Poly-beta-hydroxybutyrate/calcium polyphosphate (PHB-CaPolyPi) complexes exist as labile quasi-crystalline structures in bacterial plasma membranes. The composition, structure, and distribution of the complex suggest it may play a role in the regulation of intracellular calcium and in calcium signaling. The importance of these functions led to this investigation of the occurrence of PHB-CaPolyPi complexes in eukaryotes. A variety of plant and animal systems were analyzed and all were found to contain PHB associated with CaPolyPi. The intracellular location of the complex in bovine liver was primarily the mitochondria and microsomes, with smaller amounts in the plasma membranes. Eukaryotic PHB had the same narrow range of chain lengths (120-200 subunits) as PHB in bacterial membranes, and was associated with PolyPi of somewhat greater length (170-220) than the bacterial counterpart (130-170).