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Motor function in squid (Loligo) mantle reflects the highly coordinated activity of two motor pathways associated with giant and non-giant motor axons that respectively produce all-or-none and graded contractions in mantle muscle. Whereas both types of axons innervate circular mantle muscle fibers, precise nerve-muscle relationships remain unclear. Are squid like most invertebrates, in which single muscle fibers receive dual innervation from giant and non-giant motor axons, or is squid mantle configured more like vertebrates, in which parallel motor axon systems innervate distinct fast and slow muscle fibers? In this report, we describe giant and non-giant motor pathways that appear to control different pools of circular muscle fibers in squid. A subset of circular muscle fibers possesses large Na currents, and these fibers are proposed to employ Na-dependent action potentials to produce fast, all-or-none muscle twitches associated with giant axon stimulation., Fast and slow motor systems with diverse properties occur in many taxa (1). Studies on arthropods and chordates have revealed remarkable differences in basic neuromuscular organization, but comparative knowledge of [...]