On the basis of thin-section studies of larger foraminifera in well cores and cuttings from the Amapá Formation of the Foz do Amazonas Basin nine informal biozones (LF1-LF9) have been recognized for the upper Paleocene to middle Miocene of this basin. Lepidocyclinids are the most abundant of the larger foraminifera in the formation, and the occurrence of Lepidocyclina rdouvillei in the late middle Eocene (LF4 biozone, equivalent to planktonic foraminiferal biozones P13-P14) corroborates the hypothesis of an American origin for this genus and its rapid colonization of the northeastern margin of South America in the middle Eocene. By at least middle Eocene times, strong American affinities of the larger foraminifera of the equatorial Atlantic margins of both South America and Africa had become established by eastward migration. The presence of larger foraminifera throughout the entire carbonate section in the basin suggests that there were no significant changes in the equatorial climate of this region during the Tertiary. Extinctions of some larger foraminifera in the basin were related to environmental and ecological changes associated with sea level fluctuations.