The ontogenesis and formation stages of exocrine pancreas in European sea bass (D. labrax) larvae were investigated from hatching to 40 days after hatching (DAH). Histological and enzymatical techniques were used to explain the functional development of the pancreas in D. labrax with the expression of trypsinogen activity. The incipient pancreas appeared as a lamination of the dorsal wall of the digestive tract. It was observed that the primary visible indication of exocrine cell differentiation was polarization. The first zymogen granules and pancreas with exocrine polyhedral cells appeared on 6 DAH and became abundant as a compact structure located dorsal and slightly posterior to the liver. At the same time, firstly, anus and then mouth were opened, and total lengths of larvae were determined as 3.47 +/- 0.26 mm. Until larval metamorphosis, the pancreas became diffuse, spreading throughout the mesentery enclosure, the stomach, the upper intestine and the pyloric caeca. On the other hand, zymogen granules were more numerous and larger, and a greater quantity of material was carried by the ducts, indicating an increased cellular activity. The specific activity of trypsin was determined as early as after hatching (42.54 +/- 6.8 mU/mg protein(-1)) at 4.28 +/- 0.2 mm total length of larvae and increased immediately during the following clays especially after exogenous feeding. The highest tryptic activity was detected on 30 DAH as 122.45 +/- 11.76 mU/mg protein(-1). It is concluded that exocrine pancreas organogenesis is the main critical step of the zymogen granules and trypsin activity is present as early as after hatching and continuously increasing with larval period of D. labrax.