1. 1. The effects of the amino acid, aspartate, as an allosteric modifier of pyruvate kinase (PK) were surveyed in tissue extracts from 18 species of animals. 2. 2. Aspartate was a strong activator of PK in selected tissues of gastropod and bivalves molluscs, particularly amongst anoxia-tolerant species; with one exception, a polychaete worm, aspartate had no effect on PK from non-molluscan species tested including marine, freshwater and terrestrial forms. 3. 3. Amongst gastropods, aspartate activation of PK was confined to selected soft tissues, most often the hepatopancreas; in the whelk, Busycotypus canaliculatum, hepatopancreas, gill and kidney PK all showed the effect. 4. 4. Amongst bivalves, aspartate activated PK in all tissues tested of the oyster, Crassostrea virginica, but activated only hepatopancreas PK in Mytilus edulis and foot and phasic adductor muscle PK in Mercenaria mercenaria. 5. 5. Kinetic properties of C. virginica adductor and M. mercenaria foot muscle PK were further examined; for C. virginica adductor PK aspartate (5 mM) raised enzyme maximal velocity by 2.8-fold and lowered S0.5 for P-enolpyruvate by 36%. 6. 6. Aspartate (Ka = 4.14 ± 1.15 mM) and fructose-1,6-P2 (Ka = 0.075 ± 0.005 mM) showed strong synergistic effects in their activations of C. virginica adductor PK. 7. 7. Proposed functions for the unique activation of PK by aspartate in anoxia tolerant molluscs are (a) in gluconeogenic tissues (hepatopancreas), allosteric activators (aspartate, fructose-1,6-P2) are key to regulation of the aerobic (dephosphorylated) form of PK with respect to glycolytic vs gluconeogenic carbon flux; and (b) aspartate activation of PK provides co-ordinated control of the coupled anaerobic fermentations of aspartate → succinate and glycogen → alanine.