Determining relationships between calling activity and group size for marine mammal species is challenging, in part due to difficulties in obtaining reliable independent visual censuses of animals in open waters. In this study, acoustic calling rates of eastern Pacific gray whales were measured over a 4-week period during their 2008 breeding season in the sheltered lagoon of Laguna San Ignacio of Baja Mexico. Visual counts were conducted for 6 days during the deployment. It was found that the lagoon population more than doubled over the observational period, with much of the increase occurring over a 7-day period. Acoustic data collected during those 6 days were manually reviewed to yield counts of various gray whale call types during each day. All call rates showed peaks in early morning and evening, with minimum rates generally detected in the early afternoon, a time of low ambient noise but high tourist panga activity. The number of S1-type calls counted over 24 h increased roughly as the square of the number of the animals in the lagoon, when call counts were adjusted for variations in background ambient noise levels. An exception to this trend occurred during a time of rapid population increase in the lagoon.