The age of the Otway Basin's Casterton Formation has long been a contentious issue due to a wide range of isotopic dates (Mitchell et al., 1997) and the absence of decisive key palynological tax.a (Partridge, 1994 [unpub.] cited in Mitchell et al. 1997). Most previous palynological interpretations have broadly assigned the Casterton Formation to the Retitriletes watherooensi to Ruffordia pora australiensis Spore-Pollen Zones of the Late Jurassic to Early Cretaceous (Morgan et al., 1995· Price, 2002). Where a younger (Neocomian) age has been construed Mitchell et al., (1997) ha e proposed that, despite similar lithologies the Casterton Formation as current! defined is not a chronostratigraphic unit and the Casterton Formation east of the Merino High is younger than the Casterton Formation in the west (Mitchell et al. 1997). The palynological analysis of newly collected samples from Sawpit-I in the Penola Trough demonstrates a younger age than previously determined west of the Merino High and indicates that the Casterton Formation is a chronostratigraphic unit. Although new key spore and pollen taxa that were previously unidentified from the western Casterton Formation have been recorded no significant age refinement can be obtained based on those sporomorphs alone. The first discovery of the dinoflagellate cyst Gagiella mutabilis in the Otway Basin has provided the evidence to further refine the age of the Casterton Formation. The presence of G. mutabili restricts the age of the Casterton Formation in Sawpit- I to the Valanginian Gagiella mutabilis Microplankton Zone (Backhouse, 1988).