Mutations in the Cu/Zn superoxide dismutase (SOD1) genes are present in approximately 20% of families suffering from familial amyotrophic lateral sclerosis (FALS). Results from several transgenic studies in which FALS-related SOD1 mutations have been expressed have suggested that mutant SOD1 proteins induce cytotoxicity through a toxic gain of function, although the specific mechanism of this has not been fully clarified. To investigate the mechanism of toxicity induced by the mutant SOD1 associated with FALS, we generated transgenic Caenorhabditis elegans strains that contain wild-type and mutant human A4V, G37R and G93A SOD1 recombinant plasmids. The transgenic strains expressing mutant human SOD1 showed greater vulnerability to oxidative stress induced by 0.2 mM paraquat than a control that contained the wild-type human SOD1. In the absence of oxidative stress, mutant human SOD1 proteins were degraded more rapidly than the wild-type human SOD1 protein in C.elegans. In the presence of oxidative stress, however, this rapid degradation was inhibited, and the transgenic C.elegans co-expressing mutant human SOD1 and green fluorescent proteins (GFPs) in muscle tissues demonstrated discrete aggregates in the adult stage. These results suggest that oxidative damage inhibits the degradation of FALS-related mutant human SOD1 proteins, resulting in an aberrant accumulation of mutant proteins that might contribute to the cytotoxicity.