A flock of 9 1/2-week-old commercial tom turkeys experienced high mortality after consuming a complete feed containing an unidentified toxic substance. Initially, turkeys were found dead. Clinically, the birds were calm and still but became hyperexcitable with noise. A small percentage of birds exhibited torticollis, opisthotonos, circling, ataxia, and blindness. Findings at necropsy and upon microscopic examination were bilaterally symmetrical areas of necrosis of the cerebral hemispheres in the area of the neostriatum that were well demarcated from the surrounding normal neuropil. A feeding trial with the suspect feed in twelve 4-week-old turkey hens induced clinical disease and gross and microscopic brain changes similar to those observed in the field case. Analyses for the following substances in the suspect feed were either negative or within acceptable limits: salt, selenium, furazolidone, monensin, amprolium, 3-nitro-4-hydroxyphenylarsonic acid, aflatoxin, deoxynivalenol, zearalenone, T-2 toxin, ochratoxin, fumonisin, organophosphates, chlorinated hydrocarbons, and carbamates. The toxic component of the feed remains unidentified.