Phylogenetic comparison can reveal general principles governing the organization and neuromodulation of neural networks. Suitable models for such an approach are the pyloric and gastric motor networks of the crustacean stomatogastric ganglion (STG). These networks, which have been well studied in several species, are extensively modulated by projection neurons originating in higher-order ganglia. Several of these have been identified in different decapod species, including the paired modulatory proctolin neuron (MPN) in the crab Cancer borealis [Nusbaum & Marder (1989) J. Neurosci., 9,1501-1599; Nusbaum & Marder (1989), J. Neurosci., 9, 1600-1607] and the apparently equivalent neuron pair, called GABA (gamma-aminobutyric acid) neurons 1 and 2 (GN1/2), in the lobster Homarus gammarus [Cournil et al. (1990) J. Neurocytol., 19, 478-493]. The morphologies of MPN and GN1/2 are similar, and both exhibit GABA-immunolabelling. However, unlike MPN, GN1/2 does not contain the peptide transmitter proctolin. Instead, GN1/2, but not MPN, is immunoreactive for the neuropeptides related to cholecystokinin (CCK) and FLRFamide. Nonetheless, GN1/2 excitation of the lobster pyloric rhythm is similar to the proctolin-mediated excitation of the crab pyloric rhythm by MPN. In contrast, GN1/2 and MPN both use GABA but produce opposite effects on the gastric mill rhythm. While MPN stimulation produces a GABA-mediated suppression of the gastric rhythm [Blitz & Nusbaum (1999) J. Neurosci., 19, 6774-6783], GN1/2 activates or enhances gastric rhythmicity. These results highlight the care needed when generalizing neuronal organization and function across related species. Here we show that the 'same' neuron in different species does not contain the same neurotransmitter complement, nor does it exert all of the same effects on its postsynaptic targets. Conversely, a different transmitter phenotype is not necessarily associated with a qualitative change in the way that a modulatory neuron influences target network activity.