Thermally-induced scale asymmetries in reptiles are often considered to be indicative of underlying osteological deformities that incur fitness costs. However, this typically rests on subjective plausibility arguments and anecdotal reports about links between deformities and fitness, as well as between superficial asymmetries and deeper deformities rather than on empirical data. To shed light on these issues, we used a combination of Xrays of museum specimens and locomotor performance trials of hatchlings grass snakes (Natrix natrix) incubated in the laboratory at either 25 degrees C or 31 degrees C. We found that 30% of the museum specimens with asymmetrical ventral scales also had a rib duplication on one side of the underlying vertebrae associated with a scale asymmetry. In some cases, there was also a second extra rib on the opposite side of the vertebrae. However, although there was a statistically significant association between asymmetric ventral scales and rib duplication, a relatively weak correlation coefficient (r(s) = 0.35) indicated that scale asymmetries are not a very strong predictor of rib duplication. In the performance trials, scale asymmetries had a significant effect on terrestrial but not aquatic locomotor ability of the hatchlings, and the magnitude of this effect depended on the temperature regimes that they experienced as embryos during incubation. Although both asymmetrical and normal snakes incubated at the higher temperature had more stamina than their respective counterparts incubated at the lower temperature, asymmetrical hatchlings overall became exhausted much sooner than normal snakes across the two treatments.