Delayed neurotoxic ataxia, similar to that caused by neurotoxic organophosphorous compounds, has been shown to occur in hens after oral administration of Cyanofenphos (O-ethyl-O-Cyanophenyl phenyl phosphonothionate) following either single or repeated oral doses. Axonal and myelin degeneration affecting the long tracts in spinal cord, peripheral nerves and medulla was demonstrated. The distal fibers with large diameters were particularly affected. This finding is a new contribution which has not been previously recorded. It implies that a thorough study of the structure-activity relationships of phosphonothionates regarding their delayed neurotoxic effect is warranted.