Although the Atlantic Least Tern ( Sternula antillarum antillarum ) faces the same threats that caused the California Least Tern ( S. antillarum browni ) and the Interior Least Tern ( S. antillarum athalassos ) to be declared threatened, it is considered as “Least concern” globally, mainly because of its wide geographic distribution. However, many populations are threatened and the conservation status of several others is unknown. We evaluate, for the first time, the breeding biology of a colony of Atlantic Least Terns in the southern Gulf of Mexico. During the 2010 breeding season we censused and analyzed some relevant breeding variables for the stability of a colony at Terminos Lagoon. The starting date of egg laying and breeding peak, the clutch size variation during the breeding season and the main causes of eggs loss are similar to those reported for other colonies of this species. However, we found that the size of first (A) and single (S) eggs did not change throughout the season, but second (B) eggs size decreased. The number of breeding pairs and its higher hatching success in comparison with other colonies allow us to affirm that the colony of the Terminos Lagoon must be taken into account in conservation programs of this species. These results will settle the basis to reconsider its threat status globally and to compare breeding parameters with other colonies from the region.