Serotonin (5-HT) is an important regulator of numerous aspects of ﬂatworm biology, ranging from neuromuscularfunctionto sexualmaturationandegglaying. Intheparasiticblood ﬂukeSchistosomamansoni,5-HTtargets several G-protein coupled receptors (GPCRs), one of which has been demonstrated to couple to cAMP and regulate parasite movement. This receptor, Sm.5HTRL, has been successfully co-expressed in mammalian cells alongside a luminescent cAMP-biosensor, enabling pharmacological proﬁling for candidate anti-schistosomal drugs. Here, we have utilized this assay to perform structure-activity investigations of 143 compounds containing previously identiﬁed alkaloid natural product pharmacophores (tryptamines, aporphines and protoberberines) shown to regulate Sm.5HTRL. These experiments mapped regions of the tryptamine pharmacophore amenable and intolerant to substitution, highlighting diﬀerences relative to orthologous mammalian 5-HT receptors. Potent Sm.5HTRL antagonists were identiﬁed, and the eﬃcacy of these compounds were evaluated against live adult parasites cultured ex vivo. Such structure-activity proﬁling, characterizing the eﬀect of various modiﬁcations to these core ring systems on Sm.5HTRL responses, provides greater understanding of pharmacophores selective for this target to aid future drug development eﬀorts.