In the course of our screening for new inhibitors of mammalian cell cycle, we discovered two novel compounds named tryprostatins A (1) and B (2) from the secondary metabolites of a marine fungal strain BM939. The producing strain was isolated from a sea sediment sample collected in the 760-meter-deep sea bottom of the mouth of Oi river, Sizuoka prefecture, Japan, and identified as Aspergillus fumigatus through a taxonomic study. Here we preliminarily report the isolation, structures and biological activities of 1 and 2. The structures of tryprostatins A and B, composed of a 2-isoprenyltryptophan moiety and a proline residue forming a diketopiperazine unit (distinguished from the fumitremorgin series by opening the centered heterocyclic ring at the C-N bond between positions 18 and 10), were determined by detailed analyses of their 1H and 13C NMR spectra with the aid of 2D NMR techniques including field-gradient heteronuclear multiple-bond correlation (FG-HMBC) spectroscopy. This is the first example of natural products belonging to this novel class as inhibitors of mammalian cell cycle. Tryprostatins A (1) and B (2) completely inhibited cell cycle progression of tsFT210 cells in the G2/M phase at final concentrations of 50μg/ml of 1 and 12.5μg/ml of 2, respectively. Further studies on their biological activities and mechanism of their action are currently undertaken.