The crystal structure of the non-ribosomal lipoundecapeptide tensin from Pseudomonas fluorescens has been solved as an ethyl acetate/bis-water solvate (tensin ethyl acetate dihydrate, C67H115N12O20C4H8O22H2O) to a resolution of 0.8 AÊ . The primary structure of tensin is β-hydroxydecanoyl-d-Leu-d-Asp-d-allo-Thr-d-Leu-d-Leu-d-Ser-l-Leu-d-Gln-l-Leu-l-Ile-l-Glu. The peptide is a lactone linking the Thr3 O atom to the C-terminal C atom. The stereochemistry of the β-hydroxy acid has been shown to be S. The peptide shows structural resemblance to the non-ribosomal cyclic lipopeptide fengycin from Bacillus subtilis. The structure of tensin is essentially helical (310-helix), with the cyclic peptide wrapping around a hydrogen-bonded water molecule. The lipopeptide is amphipathic in good agreement with its function as a biosurfactant.