1. The contractile sensitivity of spiny lobster (Panulirus interruptus) foregut muscles to acetylcholine and glutamate was examined. Muscles were of three types: Class I, sensitive only to acetylcholine; Class II, sensitive only to glutamate; and Class III, sensitive to both acetylcholine and glutamate. 2. Foregut muscles of Maine lobsters (Homarus americanus), west coast crabs (Cancer magister), blue crabs (Callinectes sapidus), and rock crabs (Cancer borealis and irroratus) were tested for sensitivity to acetylcholine and glutamate. In general, homologous muscles in all species showed similar contractile sensitivity to either acetylcholine, glutamate, or both, except that in Callinectes, and C. irroratus and borealis acetylcholine sensitivity in muscles of Class III was reduced or absent. 3. Both glutamate and acetylcholine produce depolarizations of similar magnitude in the same muscle fibers of dually sensitive muscles. Estimated reversal potentials obtained by extrapolation of glutamate and acetylcholine activated depolarizations are in both cases above -20 mV. 4. Acetylcholine. receptors are distributed diffusely on fibers of dually sensitive muscles while glutamate receptors are localized to discrete foci. 5. Blockade of synaptic transmission by alteration of divalent cation concentrations does not block depolarizations of acetylcholine or glutamate. 6. Cholinergic blockers do not block synaptic transmission to dually sensitive muscles, nor do they diminish glutamate depolarizations. 7. The results indicate that dually sensitive muscle fibers probably receive glutamatergic innervation while acetylcholine receptors are distributed extra-junctionally. 8. The best transmitter candidate for the LG, MG, and DG neurons of the decapod stomatogastric ganglion is glutamate. Thus, all intrinsic muscles of the decapod foregut receive glutamatergic innervation, while extrinsic muscles receive cholinergic innervation.