Soil aggregation by earthworms is important for sustainable agriculture because soil aggregates are highly permeable and retain water. Casts are a soil aggregate and indicator of earthworm activity. The effects of reduced tillage using a rotary tiller on earthworm cast productivity and yield of Chinese cabbage were investigated in volcanic-ash soil. Five tillage practices were investigated; conventional tillage (CT), no tillage (NT), and three reduced tillage practices: shallow tillage, row tillage, and shallow row tillage respectively. Cast production of Pheretima (Amynthas) carnosa, the species which dominated the study site, and yields of Chinese cabbages were measured during three crop seasons. Crop yield under the NT treatment was lower than that under the CT treatment (p<0.05) in fall 2004, while in spring 2005 Chinese cabbage under NT treatment did not head out. Yields under reduced tillage treatments tended to decrease in spring 2005, but were similar to those under CT treatment in the other two cropping seasons. Under CT treatment, the surface cast production was almost zero. Cast production under reduced tillage exceeded that under CT treatment, but was lower than that under NT treatment. Accordingly, reduced tillage can combine the ability to preserve aggregate formation by earthworms while also preventing any decline in Chinese cabbage production yield in the cropping system studied.