Life history differentiation along climatic gradients may have allowed a species to extend its geographic range. To explore this hypothesis, we compared eleven Omocestus viridulus (Orthoptera: Acrididae) populations along an altitudinal gradient from 410 to 2440 m in Switzerland, both in the field and laboratory. In situ temperature records indicated a striking decline in available heat sums along the gradient, and field populations at high altitudes reached egg hatching and adulthood much later in the year than at low elevation. The reproductive period at high altitude is thus severely limited by season length, especially during a cool year. However, controlled environment experiments revealed that intrinsic rates of embryonic and juvenile development increased with the populations’ altitude of origin. This countergradient variation is largely genetic and conforms to predictions of life history theory. No corresponding differentiation in the overwintering egg stage, a pivotal determinant of phenology, was found. This trait seems conserved within the gomphocerine grasshopper subfamily. Although we found evidence for altitudinal adaptation in development, the potential of O. viridulus to adapt to cool alpine climates appears restricted by a phylogenetic constraint.