Proteasomal dysfunction may underlie certain neurodegenerative conditions, such as Parkinson’s disease (PD). We have shown that pharmacological inhibition of the proteasome in cultured neuronal cells leads to apoptotic death and formation of cytoplasmic ubiquitinated inclusions. These inclusions stain for -synuclein and assume a fibrillar structure, as assessed by Thioflavin S staining, and therefore resemble Lewy bodies. -synuclein is thought to be a central component of Lewy bodies. Whether -synuclein is required for inclusion formation or apoptotic death has not been formally assessed. The present study examines whether -synuclein deficiency in neurons alters their sensitivity to proteasomal inhibition-induced apoptosis or inclusion formation. Cortical neurons derived from -synuclein null mice showed a similar sensitivity to death induced by the proteasomal inhibitor lactacystin compared to neurons derived from wild type mice. Furthermore, the absence of -synuclein did not influence the percentage of lactacystin-treated neurons harboring cytoplasmic ubiquitinated inclusions or alter the solubility of such inclusions. In contrast, however, ubiquitinated inclusions in -synuclein-deficient neurons lacked amyloid-like fibrillization, as determined by Thioflavin S staining. This indicates that while -synuclein deficiency does not affect the formation of ubiquitinated inclusions, it does significantly alter their structure. The lack of effect on survival in -synuclein knock-out cultures further suggests that the fibrillar nature of the inclusions does not contribute to neuronal degeneration in this model.