The coprophilic ascomycete Coniochaeta ellipsoidea DSM 13856 forms the new antibiotic coniosetin (1) in surface cultures grown on a medium containing malt extract and oatmeal. The structure of the compound C25H35NO4, MW 413, was determined by 2D-NMR and mass spectrometric studies. Coniosetin belongs to the class of tetramic acids; it consists of a substituted aliphatic bicyclic ring system linked to a tetramic acid subunit through a carbonyl center. The absolute configuration was determined by measuring its circular dichroism spectrum and comparing the data with those of equisetin. Coniosetin has a pronounced antibacterial and antifungal action, inhibiting even multi drug-resistant strains of Staphylococcus aureus at a concentration of 0.3 microg/ml, though it is inactive against Gram-negative bacteria.