A paradigm shift has recently occurred in the field of toxicology, transitioning away from traditional mammalian models in favor of time- and cost-efficient alternatives, such as in vitro systems and non-mammalian animal models, which are amenable to high-throughput screening. We have pioneered the asexual freshwater planarian, Dugesia japonica, as an alternative model for developmental neurotoxicology. Planarians have strong regenerative capabilities, wherein after decapitation, the resulting tail piece will regenerate a new head, including a brain, within 2 weeks. Moreover, planarians possess several quantifiable behaviors coordinated by distinct neuronal subpopulations, enabling testing of both adult and developing/regenerating animals with the same assays to directly compare effects on neuronal function. We have established and begun validating the planarian toxicology platform through screens testing 10-87 compounds. We demonstrate that planarians have similar sensitivity to existing alternative animal models, such as developing zebrafish. Planarians are particularly sensitive to pesticides and are good predictors of known developmentally neurotoxic pesticides, such as organophosphorus pesticides (OPs), one of the most used class of pesticides in the world. OPs are acutely toxic due to inhibition of acetylcholinesterase (AChE), leading to accumulation of acetylcholine and subsequent cholinergic overstimulation. However, growing evidence suggests that chronic, low dose exposure to OPs, particularly during development, may cause toxicity independent of effects on AChE. Alternative mechanisms of OP developmental neurotoxicity have been proposed, but direct connections between molecular/cellular defects with their functional significance have been limited using traditional models. Our planarian screening platform, on the other hand, is uniquely suited to provide the necessary link between mechanism and functional effects. First, we characterized the in vitro and in vivo properties of planarian cholinesterase and its structural and functional interactions with OPs to contextualize known OP mechanisms. Second, through a comparative screen of 6 OPs and chemicals targeting suggested alternative OP targets, including the endocannabinoid system, cytoskeleton and oxidative stress, we correlate the distinct toxicological profiles of different OPs with specific toxic pathways. Together, these studies demonstrate the utility of the planarian system to the modern toxicology pipeline, through its ability to directly connect mechanisms with their functional significance.