A new dimeric epipolythiodioxopiperazine named chetracin A with two tetrasulfide bridges was isolated from Chaetomium nigricolor and C. retardatum, and the structure (3) proposed chiefly from proton and carbon-13 nuclear magnetic resonance (¹H- and ¹³C-NMR) data was established by X-ray analysis. Two congeners of chaetocin (2) named chaetocins B and C were isolated from C. virescens var, thielavioideum, and were proved to be homologs with one disulfide and one trisulfide bridge (9) and with two trisulfide bridges (10), respectively, from spectral data, chiefly NMR and fast atom bombardment mass spectrometry/mass spectrometry (FAB MS/MS). The structureactivity relationships observed for the antibacterial activity and cytotoxicity of the related compounds are discussed.