SUMMARYObservations on the invasion and development ofHeterodera avenaein relation to take‐all infection of barley roots were made in three pot experiments. Post‐croppingH. avenaepopulations were lower on fungus‐infected plants, as a result of the production of fewer, smaller and less fecund cysts. The reduction in cyst numbers was associated with reduced larval invasion of take‐all infected roots and with the effects of the unfavourable environment of these roots on female development, possibly through its effects on sex determination and survival of the