The structures of diolmycins A1, A2, B1 and B2, novel anticoccidial agents, were determined by spectroscopic analyses. Diolmycins A1 and A2 are stereoisomers with the structure of 1-(3-indolyl)-4-(p-hydroxyphenyl)-2,3-butanediol. From the chemical synthesis of the erythro-isomer, the relative configurations of diolmycins A1 and A2 were determined to be the erythro- and threo-isomers, respectively. The stereoisomers, diolmycins B1 and B2, were also deduced to be erythro- and threo-1,4-di-(p-hydroxyphenyl)-2,3-butanediol, respectively.