Decaspirones A-E (1-5), five new compounds related to the palmarumycins, were isolated from cultures of the freshwater aquatic fungal species Decaisnella thyridioides. The known compound palmarumycin CP1 (6) was also obtained. The structures of 1-5 were determined by analysis of NMR and MS data, and their relative configurations were assigned by analysis of 1H NMR J-values and NOESY data. The structure of the lead compound 1 was confirmed by X-ray crystallographic analysis. Compounds 1-5 possess a trans ring fusion not previously reported in members of this structural class. The absolute configuration of 1 was assigned using the modified Mosher method. Compounds 1-5 showed potent antifungal and antibacterial activity.