ALS (Amyotrophic lateral sclerosis) is a fatal neurodegenerative disease, and it is believed that mutations in the Sod1 genes likely promote SOD1 (Cu, Zn-superoxide dismutase) aggregation and cause the disease. This study demonstrates that insoluble aggregates of SOD1 have amyloid-like characteristics and are toxic to cultured cells, although the morphology is mostly either fibrillar or non-fibrillar. Using AS SOD1 (C6A/C111S SOD1) mutants, we showed that SOD1 can form aggregates with or without disulfide bonds. However, electron microscopy revealed that intermolecular disulfide bonds are responsible for the maintenance of fibrillar aggregates and indicated that amorphous aggregates are generated much more frequently from AS SOD1 than fibrils. Moreover, we observed that AS SOD1 forms amorphous aggregates by a distinct pathway; elongated fibrillar aggregates are normally produced via spherical aggregates. The results in this study may be valuable for further investigation of the molecular mechanism of SOD1-associated ALS.