We tried to correlate the level of genetic introgression in native brown trout populations with some physicochemical and environmental features. Introgresión rates were calculated according the presence (assessed by PCR-RFLP as described by Mc Meel et al. 2001) of the LDH C1*90 allele in the populations, which is fixed in central European populations used for restocking, whereas LDH C1*100 is the fixed allele in wild Mediterranean populations in the area of study. Water quality variables were obtained from Confederación Hidrográfica del Ebro data in 10-year series before sampling. Physicochemical characteristics as conductivity, pH, bicarbonates, temperature and Oxygen saturation were analyzed. Average, minimum and maximum, standard deviation and coefficient of variation were calculated for each of the above parameters. Populations were classified as wild (0% introgresión), low affected (less than 10% individuals), highly affected (10-50%) and severely affected (more than 50%). We found that highly affected populations are generally associated to higher minimum temperature and pH as well as to wide oscillations in oxygen saturation levels. Success in restocking using stocks of Atlantic domestic trout is strongly influenced by environmental features. Fortunately, wild populations show a better adaptation being able to colonise the most adverse environments. Restocking policies should be fully revised to preserve wild trout populations