A new nonadride derivative, named (–)-byssochlamic acid imide (1), was isolated from the co-culture broth of two mangrove fungi (strain Nos. K38 and E33). Their structures were established by comprehensive spectroscopic analyses. Primary bioassays showed that 1 had moderate in vitro antifungal activities against Fusarium graminearum Schw. and Fusarium oxysporum Schlecht. f. sp. lycopersici (Sacc.) W. C. Snyder et H. N. Hansen, as compared with the positive control carbendazim.