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In value-based decision making, options are selected according to subjective values assigned by the indi-25 vidual to available goods and actions. Despite the importance of this faculty of the mind, the neural mech-26 anisms of value assignments, and how choices are directed by them, remain obscure. To investigate this 27 problem, we used a classic measure of utility maximization, the Generalized Axiom of Revealed Prefer-28 ence, to quantify internal consistency of food preferences in Caenorhabditis elegans, a nematode worm 29 with a nervous system of only 302 neurons. Using a novel combination of microfluidics and electrophysi-30 ology, we found that C. elegans food choices fulfill the necessary and sufficient conditions for utility maxi-31 mization, indicating that nematodes behave exactly as if they maintain, and attempt to maximize, an un-32 derlying representation of subjective value. Food choices are well-fit by a utility function widely used to 33 model human consumers. Moreover, as in many other animals, subjective values in C. elegans are 34 learned, a process we now find requires intact dopamine signaling. Differential responses of identified 35 chemosensory neurons to foods with distinct growth potential are amplified by prior consumption of these 36 foods, suggesting that these neurons may be part of a value-assignment system. The demonstration of 37 utility maximization in an organism with a nervous system of only 302 neurons sets a new lower bound on 38 the computational requirements for its execution, and offers the prospect of an essentially complete ex-39 planation of value-based decision making at single neuron …