Animals change their behaviors in response to external stimuli, and numerous neurotransmitters are involved in these behavioral changes. In Caenorhabditis elegans, serotonin (5-HT) affects various behaviors such as inhibition of locomotion, stimulation of egg laying, and pharyngeal pumping. Previous research has shown that the neural activity of the RID interneuron increases when the worm moves forward, and the RID is necessary for sustaining forward locomotion. However, the relationship between 5-HT and neural activity of RID, and how it modulates the behavior of the worm has not been investigated. In this article, we reveal the relationship among 5-HT, RID activity, and the behavior of worms using a custom-made tracking and imaging system. We simultaneously measured the neural activity of the RID and behavior in worms with three conditions: mock animals, animals pre-exposed to 5-HT, and 5-HT receptor mod-1 mutants. As shown in previous research, the neural activity of the RID increased during the transition from backward to forward, whereas it decreased during the transition from forward to backward in mock animals. These changes in neural activity were not observed in animals pre-exposed to 5-HT and mod-1 mutants. Moreover, RID activity was correlated with the velocity of the worm in mock animals. However, this correlation was not observed in animals pre-exposed to 5-HT and mod-1 mutants. Our results demonstrate that 5-HT modulates the activity of the RID interneuron, and we infer that the RID plays a role in modulating forward locomotion by changing its activity through 5-HT.