Conspecific amphibian populations may vary widely in local demography and average body size throughout their geographical range. The environmental drivers of variation may reflect geographical gradients or local habitat quality. Among fire salamander populations (), local demography shows a limited range of variation because high concentrations of skin toxins reduce mortality from predation to a minimum, whereas average adult body size varies significantly over a wide range. This study on four neighboring populations inhabiting the catchments of low-order streams in the upper middle Rhine Valley (Koblenz, Germany) focuses on the identification of local environmental drivers of variation in age and body size. I collected 192 individuals at two localities per stream, measured snout-vent length, clipped a toe for posterior skeletochronological age determination, and released salamanders in situ again. Populations were similar in age distribution. Local habitat quality accounted for a significant proportion of demographic variability, mediated by the impact of landscape-induced mortality risk, including roads and agriculture. Still, the main effect of variation in habitat quality was on adult body size, the result of growth rates of aquatic larvae and terrestrial juveniles. Larvae exposed to non-lethal heavy metal contamination in streams developed into smaller juveniles and adults than clean-water larvae, providing evidence for carry-over effects from one stage to another. The generally small average adult size in the Rhine Valley populations compared to those in other parts of the distribution range indicates the action of a still unidentified environmental driver.