The stomach, an extension of the digestive tube, is located immediately after the esophagus. The stomach is where the mechanical and chemical processing of food takes place. The inside of the stomach is lined with glandular cells. They produce pepsin and hydrochloric acid, which are necessary for the main processing, the breakdown of protein.

In gobies, carps and some other fish species the stomach is absent - this is a peculiarity of their internal structure. Food goes directly into the dilated upper intestine. It resembles the stomach in shape, but it has no characteristic glands.