Motoneurones that drive the closing of the lateral teeth during gastric mill rhythms in spiny lobsters start firing before the motoneurones that drive the medial tooth powerstroke. This has the expected behavioural interpretation that the lateral teeth must close on a food particle before the medial tooth is pulled across it. The neural basis of the teeth coordination was examined. Experiments were made during gastric rhythms in in vitro preparations comprising the stomatogastric, oesophageal and (paired) commissural ganglia. Identified neurones in the stomatogastric ganglion were polarized to study their functional effects on the phasing and amplitude of bursts in other cells. Evoked firing of the lateral teeth closer motoneurones (especially LC) would evoke a discharge in the medial tooth powerstroke (GM) motoneurones, and suppress the firing of the medial tooth returnstroke (CP) motoneurone. Therefore the coordination pathway starts directly with the lateral teeth closer motoneurones. The CI interneurone was found to be an important link in the coordination pathway. It exerted opposite effects on the medial tooth motoneurones, suppressing firing of the powerstroke GM cells while evoking bursts in the returnstroke CP cell. CI affected other features of the pattern as well. Non-spiking inhibition from the lateral teeth closer motoneurones (LC and GP) to the lateral teeth opener motoneurones (LGs) was found to occur conjointly with spike-mediated IPSPs. Hyperpolarization of the LC, GP or CI neurones could temporarily abolish the gastric rhythm, but bursting in some or all of the other cells would eventually return, although in some cases the phase pattern was altered. It appears that no individual neurone in the gastric network is necessary for rhythm production. The coordination system can be viewed as several 'levels' of synaptic connections, each level being redundant and synergistic with the others.