Ustiloxin(l), an antimitotic tetrapeptide containing a 13-membered ring including an ether linkage, was isolated from the water extract of false smut balls caused by Ustilagirroidea kens on the panicles of rice plant. The absolute structure was determined by a combination of X-ray crystallographic and amino acid analyses. Its structure and biological activity are closely related to those of phomopsin A, a mycotoxin to cause the lupinosis.