The structure of a new pancreatic lipase inhibitor, lipstatin, produced by Streptomyces toxytricini was determined as (2S,3S,5S,7Z,10Z)-5-[(S)-2-formamido-4-methylpentanoyloxy ]-2-hexyl-3- hydroxy-7,10-hexadecadienoic lactone by spectroscopic and chemical methods. Structurally lipstatin is closely related to the known esterase inhibitor esterastin. It contains a N-formyl-L-leucine side chain instead of the N-acetyl-L-asparagine in esterastin.