A comparison in developmental process of larval digestive system was made in 18 species of teleosts. Based on the morphological features of digestive system, teleost larvae may be roughly divided into two main groups. In newly hatched larvae belonging to the first group, digestive organ is scarcely formulated; in those of the second group, digestive organs have differentiated in a certain extent at this stage, for example, mouth opened, liver and pancreas differentiated, and so on. The former is observed commonly in the larvae derived from pelagic eggs, and the latter represented by those came out from demersal eggs. But there is a considerable variation in the degree of differentiation of the system at the time of hatching among the larvae from demersal eggs, and these characteristics are believed to have developed closely connected to the spawning habit of the species as well as to the behaviour of the newly hatched larvae. No marked specific difference is found as to the sequence of formation of the organs, and the system has developed to a certain level at the end of prelarval stage. Most of the larvae show the process of development as follows; differentiation of gill opening-liver-mouth; anus-pancreas; gall bladder-gill arch-oesophagus; stomach; intestine-sphinctor of posterior intestine-pancreatic duct; bile duct. Some prelarvae hatched out from demersal eggs have oral teeth, pharyngeal teeth, taste buds and goblet cells, but lacked the gastric gland and pyloric caeca. Trout and probably salmon are exceptional. It is generalized that the digestive system in prelarval stage may be regarded as the most simplified among these of adult fish.