The present study contributes to the knowledge about the distribution of the scattered and scarce populations of the threatened Iberian endemic fish species, lamprehuela (Cobitis calderoni Bacescu, 1961) within the Ebro River basin. The Erro River was thoroughly surveyed to: 1) identify and describe reaches with suitable habitat, and 2) to elaborate a detailed distribution map in order to give conservation recommendations for the species within this protected area. The length-weight relationship (a = 0.0018, b = 3.3972, r2 = 0.9483, p < 0.0001) as well as mean Fulton’ condition factor (K = 0.3477, SE = 0.0083) for C. calderoni, and its occurrence in the Irati and Ega Rivers (Ebro River basin) are reported here for the first time.