Carbon nanotubes have become widely used due to their superior physiochemical properties, and so knowledge of their toxicity has become important. In this study, the toxic effects of nanotubes on Caenorhabditis elegans have been investigated. Multi-walled carbon nanotubes were characterized by transmission electron microscopy and particle size analysis. Upon exposure to the nanotubes at concentrations of 100 to 500 mg/L for 3 to 15 days, numbers and body lengths of the nematodes were not altered, but body thickness was raised and body movements were slightly decreased. A new type of toxicity-reproduction assay was performed to see the effect of multi-walled carbon nanotubes. The circular-form aggregation of starved larvae was counted. The number of aggregations was reduced with the increasing nanotube concentration by ∼27%. The concentration-dependent reduction was confirmed by counting the eggs in a single worm which demonstrated the negative effect of multi-walled carbon nanotubes on reproduction when the nematodes were subjected to prolonged exposure.