In this article we offer some methodological considerations about the use of censuses for the study of people of African descent in Mexico during the nineteenth century. The importance of exploring the context surrounding the production of sources is underscored to develop a critical reading of the data recorded on them. Through an analysis of censuses available, the number of enslaved people residing in Mexican Texas is examined. The conclusion is that the under-recording of the enslaved population in the area was caused by clashes over the preservation or abolition of slavery between Mexican legislators and American immigrants.