Although it is widely accepted that migration by ovigerous lobsters (Homarus americanus Milne Edwards) optimizes thermal conditions for embryonic development, temperatures experienced by freely moving lobsters have never been measured. The precise thermal histories of 30 ovigerous lobsters at large in the Gulf of Maine were recorded to compare thermal conditions experienced during a brooding season. Analysis of both the temperature and movement data revealed a clear difference between lobsters smaller and larger than the size when 50% of individuals are mature (carapace length 93 mm). Although small and large lobsters ultimately experienced a similar number of degree-days above 3.4 degree C (952.8 for small and 983.6 for large) from 25 September 2002 until 27 July 2003, large lobsters and their embryos experienced less extreme and less variable temperatures. They were exposed to more gradual cooling in the fall and more gradual warming in the spring. These data, which are the first to document the seasonal temperatures experienced by ovigerous lobsters, suggest that migrations do not necessarily increase the number of degree-days experienced by developing embryos, but do reduce the variation in their thermal regime.