1. The phenology of Neophilaenus lineatus (Homoptera: Cercopidae) was studied along altitudinal and latitudinal transects in the UK. The response of the life cycle to these existing temperature gradients has been used to predict the likely effects of future climatic warming. 2. The date of spring egg hatch varied by a maximum of 4 weeks across the altitudinal gradient (440 m) and by 2 weeks in different years of the study. Autumn and winter temperatures do not determine hatching date because the eggs are in diapause during this period. It is only when the eggs terminate diapause in early spring that temperature differences between years and altitudes (or as a result of climatic warming) start to affect the date of egg hatch. 3. The delayed egg hatching with increasing altitude meant that the start of nymphal development was later at higher altitudes. The development rate of nymphs at higher altitudes was not significantly different from that at lower altitudes because the altitudinal lag in temperature was partially compensated for by the more advanced state of the season. Annual differences in spring and summer temperatures resulted in variations in nymphal development rate between years, with a 1 °C temperature rise shortening nymphal development by 3·5 days. 4. The appearance of adults was delayed by 5·6 days for every 100 m increase in altitude, largely as a consequence of the later egg hatch at higher altitudes. The later appearance of the adults at higher altitudes and in colder years, reduces the length of the potential oviposition period as females are killed by autumn frosts. Climatic warming would expand the length of the oviposition period and thus increase the upper altitude limits of N. lineatus. 5. A 4·5 ° latitude difference within the UK appeared to have little effect on the timing of development in N. lineatus, possibly as a result of a geographical cline in temperature or photoperiod response. 6. Annual insects, such as N. lineatus, are likely to show a relatively small geographical displacement as a consequence of climatic warming. This is because of the temperature mitigating responses of diapause and the relative stability of the length of the development period over wide altitudinal and latitudinal ranges.