The National Museum of Natural Sciences (MNCN, for its acronym in Spanish) Ichthyology collection is the largest among the Museum’s Vertebrate collections. It holds more than 30,000 lots, with over 375,000 specimens, which also makes it the nation’s largest collection of its kind.

The holdings contain specimens of practically every large group of fauna which is cataloged as fish from a classical point of view; specimens of lamprey, chimaeras, sharks, rays, sturgeons, lungfish, bichirs, alligator gars, and a myriad of modern bony fishes are readily available to visitors. There are representatives of roughly 2,350 different species, of more than half of the 482 families of living fish acknowledged by Nelson (1994). We house freshwater and marine taxa Spanish species, and a considerable representation of worldwide species from Europe, America, Africa and Asia. The collection is currently recognized as an international resource for research in continental ichthyofauna of some geographical areas such as the north of Africa, Central America or, more particularly, the Mediterranean basin.

Some of the holdings of the collection – namely the type specimens Squalius palaciosi (Doadrio, 1980),  Squalius carolitertii (Doadrio, 1984),  Chondrostoma turiense (Elvira, 1987),  Cobitis vettonica (Doadrio & Perdices, 1997), and Aphanius baeticus (Doadrio, Carmona & Fernández Delgado, 2002) - epitomise the important changes that have been lately introduced in the taxonomy of the Iberian ichthyofauna.

Most of the specimens are stored in ethyl alcohol, but some are cleared or stained; the collection is strong in disarticulated skeletons, and it contains old collections of traditionally mounted (naturalized) specimens, as well as several skeleton pieces and mounted skeletons of great historical importance.

While this is obviously a historically valuable collection, it has also grown by ca. 500% over the last 20 years, which ranks it amongst the most modern collections of its kind. Most holdings are well-dated and stored. Roughly 87% of the collection is classified under the taxonomic level of species or genus, a 10% is classified into families, and the remaining 3%, into different orders.