Reelin (RELN) is an extracellular matrix protein largely related with laminar organization in several brain areas. The development of RELN immunoreactivity in the retina and the optic tectum of the brown trout are analyzed with a monoclonal (142) antibody against RELN whose suitability has been ascertained by western blot. In the retina of embryos and alevins, RELN immunoreactivity is detected in cells of the ganglion cell layer (GCL) and inner nuclear layer (INL), and in the inner plexiform layer (IPL), where it appears as "diffuse" material confined to the ON-sublayer. In juveniles, RELN expression becomes restricted to a stripe of cells in the INL. RELN-immunoreactive (RELN-ir) cells are absent from the outer nuclear layer (ONL) at any developmental stage. The developmental pattern of RELN expression in the trout retina shows many similarities with that of amniotes: (a) RELN expression parallels the vitreal to scleral progression of differentiation of the retina and, within each cell layer, RELN immunoreactivity appears confined to a subpopulation of postmitotic cells; (b) at early stages RELN expression is exclusively observed in the central retina and as maturation progresses from the center to the periphery, more RELN-ir cells are observed following the same spatial pattern. Differences with amniotes are noted regarding the absence of RELN expression in the GCL and INL in adulthood, and in the ONL at any developmental stage. In the optic tectum (OT) of trout, as in amniotes, RELN immunoreactivity increases within specific cell layers as lamination proceeds, and decreases when it is complete, except in the stratum opticum (SO), where RELN-ir cells are observed throughout life. Time-course expression of RELN in the OT suggests a role in the early modeling of synaptic contacts and the accommodation of new retinal arriving axons throughout life.