BIOLOGICAL CHARACTERZATION OF COTTUS SP. POPULATIONS (ACTINOPTERYGII, COTTIDAE) INGERMANY. The bullhead, in all its geographical area, shows inter–populations variations. In the zone of our studies, in Germany, the four Cottus sp.populations studied, in Ottersbach, Logebach, Giertshagener Bach and in Krabach,show many different ecological requirements and each of these populations is characterized by its own average standard length (Ls). The comparison of this one two by two, shows significant differences (to 5 %), except between the populations of Krabach and Giertshagner Bach where there are not significant differences (to 1%). The age of the bull-head in the four studied habitats varies from 0+to 4+years and the Ls fluctuations amplitude, for the same age group,are small: the different ages groups recover each others and the growth of length is strongly linked to an increase of age. The studied Cottus sp. populations present the same growth with a slight advantage for the oldest fish of Logebach.In this one, the proportion of the females is raised 71,7 % followed by the population of Krabach (71,1 %) and Otters-bach (66 %). On the other hand, in the Giertshagener Bach population, the sex–ratio is balanced (47,9 % of males and 52,1 % of females). Key Words: Cottus sp., population, length, weight, otolith, growth, sex–ratio, Germany.