Mammalian Delta9 stearoyl-CoA desaturase 1 (SCD1) is a key enzyme in the biosynthesis of mono-unsaturated fatty acids in the endoplasmic reticulum (ER). It is a short-lived multispanning ER membrane protein, reported to be degraded by the ubiquitin-proteasome-independent pathway. We have examined SCD1 protein degradation using cultured mammalian cells. Exogenously expressed SCD1 in CHO-K1 cells was localized to the ER and turned over with a half-life of approximately 3 hours. Unexpectedly, proteasome inhibitors increased the half-life of SCD1 to approximately 6 hours. Endogenously expressed SCD1 in adipocyte-differentiated NIH 3T3-L1 cells was also rapidly degraded in a proteasome inhibitor-sensitive manner. In the presence of proteasome inhibitors, polyubiquitylated SCD1 accumulated in the ER and interacted with AAA-ATPase p97, which is involved in ER-associated degradation (ERAD). The 66-residue N-terminal segment carrying the PEST sequence is mainly responsible for SCD1 degradation and this segment induced instability in an otherwise stable ER membrane protein. Furthermore, SCD1 was degraded constitutively irrespective of the cellular levels of unsaturated fatty acids, which strictly regulate SCD1 gene expression. These findings indicate that the ubiquitin-proteasome-dependent ERAD system is also involved in constitutive SCD1 degradation.