Traditional tree fallows have been abandoned on the western coast of the Reunion Island because of the increasing need for cultivated land. Soil fertility is no longer restored and crop yields have decreased drastically. The leguminous plant, Lotus uliginosus (trefoil), used as a cover crop, has made possible the control of erosion, the restoration of soil macrofauna, especially earthworms, and the increase in crop yields. When trefoil was associated with earthworms (Amynthas corticis), the densities of maize, the yields of maize stalk and dry matter, the yield of trefoil fodder dry matter, and the biomass and respiratory activity of soil microflora were considerably increased. The combined effects of their association led to a significant decrease in populations of the plant-parasitic nematode, Pratylenchus vulnus, in maize roots, and in the population of borers. Some soil chemical features were modified.