Ocean acidification is an important consequence of rising levels of atmospheric CO2. The chemistry of acidification is, however, general and may disturb pH in terrestrial systems. The present study examines the effects of rising CO2 on insect eggs, which may be vulnerable to acidification because they are small, have (at least initially) poorly developed physiological systems, and support important developmental events. Newly-laid eggs of the moth Manduca sexta are exposed to levels of CO2 between 0 and 2200 p.p.m., in air, and effects on yolk pH, total developmental time, and survival are measured. Altered CO2 has no effect, over several hours, on the pH of egg yolk, suggesting that yolk fluids are well buffered. By contrast, there is a large developmental change in yolk pH. Eggs exposed to eight different levels of CO2 for the duration of development show a small but significant parabolic response in development time. Eggs develop fastest at intermediate levels of CO2, between 400 and 1200 p.p.m., and slower at 0, 1600 and 2000 p.p.m. These results suggest that future rises in CO2 may not have strong direct effects on insect development.