The organization of the primary visual system of the larval trout (Salmo irideus) was studied at different stages of development: 5 days before hatching, 1 and 2 months post-hatching, by Fink and Heimer's method and radioautography. After ablation of the right retina or injection of 14C Proline (0,05 microCi in 0,5 microliter) in the right eye, animals were kept at 13 degrees C for 6 hours to 7 days and fixed according to the technic used. We also stained transverse sections of larval stages (14 days before hatching to hatching stage) with Bodian's method. The results show that the retino-tectal fibers are precocious. They are present five days before hatching. The retino-thalamic projections appear at hatching. The geniculate nucleus has a mesencephalic origin. The radioautograms show this structure at hatching. The optic tectum differentiates progressively latero-medially and from the anterior level to the posterior level. Among the 3 correspondent optic tectal layers of the adult (superficial, intermediate and deep), the second is present at hatching while the two others appear progressively between one and two months. A comparison of the fish brain with that of other vertebrates is difficult. We try to discuss some problems of homology (geniculate nucleus, pretectum, accessory optic centre).