Pollen profiles of two cores from deep on the seaward flank of the Middle America Trench are used to interpret the Holocene climatic history of adjacent southwestern Mexico. Four broad pollen zones are distinguished, which carbon-14 analysis dates to 8000-9000 yrs B.P. The total pollen and spores per gram of sediment, together with the abundance and relative percentages of certain pollen types (pine, oak, alder, fir) and altered specimens, are interpreted to reflect variations in moisture and temperature during the Holocene in cordilleran Mexico. Source vegetation and the proportionate roles of transportation to the sea by streams and winds are considered to be the principal factors in emplacing the pollen zones. Large numbers of specimens per gram sediment, diversity of types, higher percentages of altered grains, and abundance of oak, alder, and fir pollen are presented as evidence for greater available moisture and increased stream activity on the western slopes of the Sierra Madre del Sur. On the other hand, fewer specimens and the preponderance of relatively few types (mostly pine pollen and fern spores) are considered to represent diminished stream activity. The oldest zone (Zone IV) represents a climatic phase which varied from cool-dry to cool-moist. Zone III is interpreted to reflect the period of maximum warmth and moisture. The next higher zone reflects the driest climatic phase represented in the cores. The upper-most zone (Zone I) marks the return to more moist conditions.