The age and growth of the Iberian cyprinid Barbus bocagei Steindachner 1865, were studied in six different streams in the River Tagus basin. The fish were aged by scale reading and their lengths were back-calculated. Age structures were similar at four sites (R. Ocreza, R. Rio Frio, R. Sever and R. Canha), where six to seven age classes were present. In contrast, 10 age groups were detected in the R. Erges and 11 in the R. Nabao. The slopes (b) of the length–weight regressions did not differ significantly between the six populations. The growth of the populations was compared with that available throughout the study area (another two sites). Classification analysis of the lengths-at-age by sites matrix revealed three major groups (by decreasing order of growth rate): 1, Belver Reservoir; 2, R. Tagus, Nabao and Erges; 3, Rio Frio, Sever, Canha and Ocreza. We hypothesize that highly stressful conditions in the summer diminish growth rates and longevity in the Rio Frio, Canha and Ocreza rivers and that lower temperatures all year round could limit growth in the Sever. Higher growth rates were observed in warm-water streams with more stable environmental conditions (group 2) and in a highly productive lentic system (group 1). Among the sites that were studied, the one that revealed the highest growth performance by B. bocagei was the most polluted, the R. Nabao. This offers evidence of the species plasticity and of its successful adaptation to streams displaying significant degradation in water quality.