Variability is a prominent feature of behavior, and an active element of certain behavioral strategies. To understand how neuronal circuits control variability, we examined the propagation of sensory information in a chemotaxis circuit of C. elegans where discrete sensory inputs can drive a probabilistic behavioral response. Olfactory neurons respond to odor stimuli with rapid and reliable changes in activity, but downstream interneurons respond with a probabilistic delay. The interneuron response to odor depends on the collective activity of multiple neurons – AIB, RIM, and AVA -- when the odor stimulus arrives. These three neurons participate in an ongoing synchronously fluctuating internal state that drives behavior and has a very probabilistic response to odor. However, certain activity states respond to odor more reliably. Artificially generating these activity states by modifying neuronal activity increases the reliability of odor responses in interneurons and the reliability of the behavioral response they encode. The integration of sensory information with network state may represent a general mechanism for influencing perception and generating variability in behavior.