Thermotactic behavior in Caenorhabditis elegans is sensitive to both a worm’s ambient temperature (Tamb) and its memory of the temperature of its cultivation (Tcult). The AFD neuron is part of a neural circuit that underlies thermotactic behavior. By monitoring the fluorescence of pH-sensitive green fluorescent protein localized to synaptic vesicles, we measured the rate of the synaptic release of AFD in worms cultivated at temperatures between 15 and 25°C, and subjected to fixed, ambient temperatures in the same range. We found that the rate of AFD synaptic release is high if either Tamb >Tcult orTamb <Tcult, but AFD synaptic release is low if Tamb ≅Tcult. This suggests that AFD encodes a direct comparison between Tamb andTcult.