The development of the subarcuate fossa and the cerebellar paraflocculus was studied in an ontogenetic series of Monodelphis domestica Wagner, 1842 The spatial relation between these structures was examined qualitatively in adult specimens of several marsupial taxa. The fossa is first formed without participation of the cerebellar paraflocculus, which fills the fossa first fully and then partially later in development. The correlation between the size of the petrosal lobule of the paraflocculus and the subarcuate fossa in adults is weak. The volume of the subarcuate fossa was measured in 68 specimens representing 19 species of recent marsupials. Its size is negatively allometric with respect to skull size. The didelphids examined ('large opossums') have relatively smaller subarcuate fossae than the other marsupials examined, and Sarco- philus laniarius is the major outlier, with a very small fossa. Loss of the subarcuate fossa has occurred at least twice in metatherian evolution (some sparassodonts and wombats). All marsupials examined to date, with the exception of wombats, have a differentiated petrosal lobule of the paraflocculus.
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