We conducted structured surveys using photographs of 88 fish species grouped by commercial and non-commercial relevance to analyze the characteristics of the folk taxonomy of artisanal fisheries in the lower La Plata river basin (Argentina). The photographs were shown to 60 artisanal fishers from the Paraná and Uruguay Rivers. Out of a set of 332 folk names recorded, monotypic names (79%) prevailed and over-differentiation (68%) was the most frequent correspondence among generic folk names. Morphology (68%) and ecology (29%) were the most common categories related to specific folk nomenclature. The number of folk names per species was lower amongst the most relevant commercial species; the percentage of species recognized decreases inversely with species relevance but increases with species body size. A comparison of species similarity based on presence/absence data in five fishing sites showed a more accurate picture of species distribution according to fishers' knowledge than that based on the scientific literature. Our observed results suggest that ethnoichthyological information can be successfully applied to improve fish conservation and fisheries management; it also provides new insights on species abundance and distribution. We conclude that the folk taxonomy method is a valuable tool for long-term monitoring research programs oriented to species conservation and resource management.