In our continuing search for antitumor metabolites from microorganisms isolated from marine organisms,1) we have isolated a strain of Fusarium chlamydosporum OUPS-N124 as producing microorganisms of cytotoxic materials from the marine alga Carpopeltis affinis. Investigation for metabolites of this fungal strain has led to the isolation of two new sulfur-containing dioxopiperazine derivatives (1 and 2), designated fusaperazines A and B, respectively, in addition to four known compounds (3-6). We report herein the isolation and stereostructures of 1 and 2 (Fig. 1), and the absolute configuration of the known compounds 3 and 4, previously undetermined.