Two novel diketopiperazines named tryprostatins A and B and a new natural product belonging to the diketopiperazine series, designated as demethoxyfumitremorgin C, together with four known diketopiperazines, fumitremorgin C, 12,13-dihydroxyfumitremorgin C, fumitremorgin B and verruculogen, are new M phase inhibitors of the mammalian cell cycle, which were isolated from the secondary metabolites of Aspergillus fumigatus. The structures of tryprostatins A, B and demethoxyfumitremorgin C were determined mainly by the use of spectroscopic methods especially by detailed analyses of their 1H and 13C NMR spectra with the aid of 2D NMR techniques including pulse field gradient heteronuclear multiple-bond correlation (PFG-HMBC) spectroscopy. Their absolute configurations were determined on the basis of the optical rotational values and CD spectra.