The digestive system of first stage Argulus japonicus larvae is described following reconstruction from serial sections. The morphology of the digestive tract is similar to adult branchiurans. It consists of an esophagus, esophageal funnel, anterior midgut (crop), midgut diverticula, posterior midgut (intestine) and a hindgut. However, the epithelial cells lining the midgut of newly hatched larvae contain large amounts of yolk. The midgut diverticula are less ramified than in the adult and arise from the anterior midgut as two main arms which branch anteriorly and posteriorly into the carapace. The posterior midgut is lined with large swollen cuboidal epithelium with large vacuoles and a ciliated border, whereas the adult posterior midgut is lined by large papilliform cells. Argulus japonicus larvae survive a day after hatching without nutrition from a host. First stage larvae feed mainly on host epithelial cells and mucus. There was no blood observed in the lumen of the digestive system.