The immunobiology of marsupial IgE is poorly understood. As a first step towards the development of immunological reagents for marsupials and to obtain a further understanding of immunoglobulin evolution, a brushtail possum (Trichosurus vulpecula) mesenteric lymph node cDNA library was screened for the heavy chain constant region of IgE (Cepsilon), using a partial Cepsilon probe from the American marsupial, Monodelphis domestica. The cDNA sequence for T. vulpecula Cepsilon was determined and found to be most similar to the M. domestica Cepsilon sequence [(76%) at the amino acid level]. T. vulpecula Cepsilon has amino acid sequence similarities ranging from 43-52% with various eutherian Cepsilon sequences. The secondary structure of T. vulpecula Cepsilon, based on loops formed by internal disulfide bonds, more closely resembles rodent Cepsilon than the American marsupial sequence.