The purpose of this investigation is to characterize parvalbumin-immunoreactive (IR) neurons in the inner nuclear layer (INL) of zebrafish retina through immunocytochemistry, quantitative analysis, and confocal microscopy. In the INL, parvalbumin-IR neurons were located in the inner marginal portion of the INL. On the basis of dendritic stratification in the inner plexiform layer (IPL), at least two types of amacrine cells were IR for parvalbumin. The first one formed distinctive laminar tiers within s4 (PVs4) of the IPL, and the second within s5 (PVs5). The average number of PVs4 cells was 8263 cells per retina (n=3), and the mean density was 1671cells/mm(2). The average number of PVs5 cells was 1037 cells per retina (n=3), and the mean density was 210cells/mm(2). Quantitatively, 88.9% of anti-parvalbumin labeled neurons were PVs4 cells and 11.1% were PVs5 cells. Their density was highest in the midcentral region of the ventrotemporal retina and lowest in the periphery of the dorsonasal retina. The average regularity index of the PVs4 cell mosaic was 4.09, while the average regularity index of the PVs5 cell mosaic was 3.46. No parvalbumin-IR cells expressed calretinin or disabled-1, markers for AII amacrine cells, in several animals. These results indicate that parvalbumin-IR neurons in zebrafish are limited to specific subpopulations of amacrine cells and the expressional pattern of parvalbumin may not correspond to AII amacrine cells in several other animals. Their distribution suggests that parvalbumin-IR neurons are mainly involved in ON pathway information flow.