Life history traits of the vulnerable Iberian loach Cobitis calderoni were studied in the River Lozoya, a regulated river in Central Spain. A total of 163 specimens were collected on five occasions, from April to December 1990. In general C. calderoni grew isometrically and a length-frequency analysis showed 4–5 age classes. Parameters fitted to the von Bertalanffy growth model showed variation in growth rates between the sexes. The gonadosomatic index for both sexes were similar and varied among months, with the highest values in April, just before the onset of the spawning season in Spring.