In the current theory of learning, the reward prediction error (RPE), the difference between expected and received reward, is thought to be a key factor in reward-based learning, working as a teaching signal. The activity of dopamine neurons is known to code RPE, and the release of dopamine is known to modify the strength of synaptic connectivity in the target neurons. A fundamental interest in current neuroscience concerns the origin of RPE signals in the brain. Here, we show that a group of rat striatal neurons show a clear parametric RPE coding similar to that of dopamine neurons when tested under probabilistic pavlovian conditioning. Together with the fact that striatum and dopamine neurons have strong direct and indirect fiber connections, the result suggests that the striatum plays an important role in coding RPE signal by cooperating with dopamine neurons.