The development of the gilthead sea bream, Sparus aurata, endocrine pancreas was studied from hatching to 114 days, using immunocytochemical techniques. Bonito insulin (INS)-, synthetic somatostatin-14 (SS-14)-, salmon somatostatin-25 (SS-25)-, mammalian somatostatin-28 (1-12) (SS-28)-, porcine glucagon (GLU)-, glucagon-like peptide-1 (1-19) (GLP-1)-, synthetic porcine peptide tyrosine tyrosine (PYY)-, and neuropeptide Y (NPY)-like immunoreactivities were demonstrated. The different types of endocrine cells appear at distinct stages of development and differ in their arrangement. The coexistence of INS and SS-25 immunoreactivities was demonstrated in the cells of one strand or primordial cord and a primordial islet that appeared close to the dorsal epithelium of the anterior region of the undifferentiated gut or next to the gut at hatching and one day after hatching, respectively. INS- and SS-25- immunoreactive (ir) cells were located in the core and at the periphery of the single islet found in 2-day-old larvae, while SS-28-ir cells were found in the single islet in 4- to 11-day-old larvae. GLU/GLP-1-ir cells were located next to the outer SS-25-ir cells in the single islet of 12- and 16-day-old larvae. SS-14/SS-25- and SS-14/SS-28-ir cells were detected in the outer region and in the inner area of the single islet, respectively, in 17- to 23-day-old larvae. One big islet and several small islets and isolated or clustered cells next to the pancreatic duct were present in 24- and 25-day-old larvae. The islets were similar in cell composition to the single islet seen in the previous stage, while the isolated and grouped cells showed the coexistence of INS and diverse SSs immunoreactivities. Nerve fibers showing PYY immunoreactivity were identified in the islets from 17 days onwards. In 30- to 44-day-old larvae, GLU and NPY immunoreactivities coexisted in a few cells at the periphery of some small islets. PYY-ir cells were first detected at day 51. One big islet, several intermediate islets and numerous small islets were present from 51-day-old-larvae to juveniles. GLU was colocalized with PYY and NPY in a few cells in a small peripheral area in the big islet and a few intermediate islets. The outer region of small islets and other intermediate islets showed the complete coexistence of GLU, PYY, and NPY.