The nematode C.elegans, with its 302 neurons and abundance of genetic, laser ablation, electrophysiological and imaging tools, is a compact and attractive system for neural circuit analysis. An understanding of the functional dynamics of neural computation requires physiological analyses. We undertook the first characterization of transfer at central synapses in C.elegans. To achieve this we employed optical stimulation techniques using channelrhodopsin-2, and combined this with whole-cell patch clamp electrophysiological recording techniques. We show that the synapse between AFD and AIY, the first stage in the thermotactic circuit, exhibits excitatory, tonic and graded release. The gain at the synapse was low (