SUMMARYThe effects of invasion by the cereal cyst nematode,Heterodera avenae, on the root growth of susceptible and resistant oat seedlings were similar. Nematode invasion directly inhibited root extension and lateral root production, which was dependent upon the developmental status of the invaded root tissue and the nematode density cm‐1root. Both responses are caused by mechanical damage due to nematode invasion leading to decreased top growth in the host. Reduced root development, decreased efficiency of water and nutrient uptake in invaded plants being indirect effects of nematode invasio