Clinical, necropsy, bacteriologic, parasitologic, histopathologic, toxicologic and animal inoculation studies suggest that organochlorine (PCB, dieldrin and DDE) poisoning was an important factor in causing deaths of free-flying ring-billed gulls (Larus delawarensis) in southern Ontario in 1969 and 1973. The brains of gulls dying with clinical signs of neurologic involvement, and dead gulls with no other apparent cause of death, contained organochlorine residues of significantly greater levels than those found in healthy gulls shot for comparison.