Teleost fin rays regenerate when they are cut, forming a distal proliferating blastema that restores the entire structure in a few weeks. In the present study we have analysed the histochemical properties of the cells and the extracellular matrix of regenerating fins in two teleost species, Carassius auratus and Brachydanio rerio. Different cell types and states have been defined. Four epidermal cell types, i.e. peridermal, epidermal, basal epidermal and mucous cells, display different histochemical and lectin binding properties, depending on their distribution in the regenerative tissue. Blastema differentiates into two cell types: scleroblasts and fibroblasts. The scleroblasts stem from blastemal cells that have contacted the lateral palisade of actinotrichia and begun to release lepidotrichial material beneath the subepidermal basement membrane. The fibroblasts differentiate from blastemal cells in the centro-proximal region of the blastema. Both cell types pass through transient states. Blastemal cells showed increased affinities for Concanavalin agglutinin, Ricinus communis agglutinin, toluidine blue and aldehyde-fuchsin, as they differentiated into scleroblasts and fibroblasts. Three different regions are histochemically distinguishable in the precalcified lepidotrichial hemisegment, reflecting different degrees of maturation: mature matrix, central band and the peripheral (immature) region contacting scleroblasts. Maturation of lepidotrichial matrix is characterized by the lost of affinity for lectins and strong positivity for silver methenamine. A catalogue of position-dependent histochemical affinities defining cell diversity in the regenerating fin of teleosts is proposed in order to correlate changes with the underlying regeneration mechanisms.& b d y :