Ubiquitin-dependent proteolysis plays a pivotal role in stress responses. To investigate the mechanisms of these cellular processes, we have been studying Schizosaccharomyces pombe mutants that have altered sensitivities to various stress conditions. Here, we showed that Lub1, a homologue of Ufd3p/Zzz4p/Doa1p in budding yeast, is involved in the regulation of ubiquitin contents. Disruption of the lub1+ gene resulted in monoubiquitin as well as multiubiquitin depletion without change in mRNA level and in hypersensitivity to various stress conditions. Consistently, overexpression of genes encoding ubiquitin suppressed the defects associated with lub1 mutation, indicating that the phenotypes of the lub1 mutants under stress conditions were due to cellular ubiquitin shortage at the posttranscriptional level. In addition, the lub1-deleted cells showed aberrant functions in ubiquitin/proteasome-dependent proteolysis, with accelerated degradation of ubiquitin. Also Cdc48, a stress-induced chaperon-like essential ATPase, was found to interact with Lub1, and this association might contribute to the stabilization of Lub1. Our results indicated that Lub1 is responsible for ubiquitin homeostasis at the protein level through a negative regulation of ubiquitin degradation.