Aspergillus sp. No. FA2692, isolated from a bark of Quercus mongolica collected in Shiga-kogen, Nagano Prefecture, Japan, was found to produce a new antibacterial and antifungal compound designated as BU-4704. The structure of BU-4704 was determined to be l -(4-methoxyphenyl)-4-(4-hydroxysulfonyloxyphenyl)-2,3-diisocyano- 1 ,3-butadiene, the xanthocillin class of antibiotics1 ~3). This paper deals with the structure and antimicrobial activity of BU-4704.