The activity of the crayfish abdominal postural motoneurons and their associated neurons (the accessory neuron(s) and the MRO(1)) were examined with the aid of techniques for the analysis of simultaneously recorded spike trains. A means of reliably identifying the spikes of the individual motoneurons based on their relative axon conduction velocities is presented. The analyses show that: 1) the large, phasically active synergist motoneurons innervating muscles producing the same movement show a marked similarity in their average responses, which is independent of the input source; 2) the small, tonically active and the middle-sized, tonicphasic synergist motoneurons innervating the same muscle and similar synergist motoneurons innervating antagonistic muscles are coordinated entirely by premotoneuron connections; 3) the accessory neuron is coordinated in its activity with the phasically active flexor excitor motoneurons and the extensor inhibitor motoneuron and thereby functions as a flexor synergist; and 4) the simultaneous presentation of flexion-producing and extension-producing inputs to the postural system results in a reciprocal oscillation in flexor-extensor motoneuron output. The functional significance of these results with respect to the operation of the postural system are discussed.