Stable-isotope profiles through successive growth increments of Miocene bivalves from Western Amazonia reveal paleobiological characteristics, such as biotope preferences, longevity, and ontogenetic development. Two groups of bivalves, a fluvial and a lacustrine group, are recognized. The fluvial group is composed of pearly freshwater mussels whose isotope profiles are characterized by clearly recognizable cyclicity of considerable amplitude, similar to those from pearly freshwater mussels studied from the modern Amazon region. The lacustrine group, dominated by pachydontine bivalves, is characterized by very low amplitude and irregular isotope signals. Additional trace element analyses show seasonal variation in phase with stable isotope cycles. Seasonal stable isotope cycles show growth rates and longevity in bivalves from the fluvial group but are not clearly expressed in the lacustrine group. The Miocene Pebas ecosystem of Western Amazonia can be characterized as a wetland system of connected shallow lakes, swamps, and tributaries. The stable isotope signatures of fluvial bivalves are good recorders of prior seasonal climate changes, in contrast with those of lacustrine bivalves. © 2005 Elsevier Ltd. All rights reserved.