Earthworms have been shown to increase plant growth in 75% of the experiments that have compared plant growth in their presence and absence. However, the relative importance of the different mechanisms advanced to explain such a stimulatory effect has never been tested. In a laboratory experiment, we observed increased growth of rice plants in the presence of earthworms (Millsonia anomala, Megascolecidae) and demonstrated that enhanced nitrogen release (generally considered as the principal mechanism involved in earthworm positive effect on plants) was not responsible for this result: earthworms had the same stimulatory effect on plant growth (+20%) irrespective of whether the soil (provided with different amounts of mineral-N fertilizer) was either N-limited or N-saturated. We discuss alternative explanations for the observed variations in rice production.