The marine-derived fungus Stachylidium sp. was isolated from the sponge Callyspongia sp. cf. C. flammea. Culture on a biomalt medium supplemented with sea salt led to the isolation of two new, most unusual N-methylated peptides, i.e., the tetrapeptides endolide A and B (1 and 2). Both of these contain the very rare amino acid 3-(3-furyl)-alanine. In radioligand binding assays endolide A (1) showed affinity to the vasopressin receptor 1A with a Ki of 7.04 μM, whereas endolide B (2) exhibited no affinity to the latter receptor, but was selective toward the serotonin receptor 5HT2b with a Ki of 0.77 μM.