C-type allatostatins (AST-Cs) are pleiotropic neuropeptides that are broadly conserved within arthropods; the presence of three AST-C isoforms， encoded by paralog genes， is common. However， these peptides are hypothesized to act through a single receptor， thereby exerting similar bioactivities within each species. We investigated this hypothesis in the American lobster， Homarus americanus， mapping the distributions of AST-C isoforms within relevant regions of the nervous system and digestive tract， and comparing their modulatory influences on the cardiac neuromuscular system. Immunohistochemistry showed that in the pericardial organ， a neuroendocrine release site， AST-C I and/or III and AST-C II are contained within distinct populations of release terminals. Moreover， AST-C I/III-like immunoreactivity was seen in midgut epithelial endocrine cells and the cardiac ganglion (CG)， whereas AST-C II-like immunoreactivity was not seen in these tissues. These data suggest that AST-C I and/or III can modulate the CG both locally and hormonally; AST-C II likely acts on the CG solely as a hormonal modulator. Physiological studies demonstrated that all three AST-C isoforms can exert differential effects， including both increases and decreases， on contraction amplitude and frequency when perfused through the heart. However， in contrast to many state-dependent modulatory changes， the changes in contraction amplitude and frequency elicited by the AST-Cs were not functions of the baseline parameters. The responses to AST-C I and III， neither of which is COOH-terminally amidated， are more similar to one another than they are to the responses elicited by AST-C II， which is COOH-terminally amidated. These results suggest that the three AST-C isoforms are differentially distributed in the lobster nervous system/midgut and can elicit distinct behaviors from the cardiac neuromuscular system， with particular structural features， e.g.， COOH-terminal amidation， likely important in determining the effects of the peptides. NEW & NOTEWORTHY Multiple isoforms of many peptides exert similar effects on neural circuits. In this study we show that each of the three isoforms of C-type allatostatin (AST-C) can exert differential effects， including both increases and decreases in contraction amplitude and frequency， on the lobster cardiac neuromuscular system. The distribution of effects elicited by the nonamidated isoforms AST-C I and III are more similar to one another than to the effects of the amidated AST-C II.