Morphology-guided cell-based screening of the extract of a Mycale sp. marine sponge led to the isolation of two trisoxazole macrolides, miuramides A (1) and B (2), which induced characteristic morphological changes in 3Y1 cells. The structure of 1 including absolute configuration was elucidated by a combination of the analysis of spectroscopic data, derivatization, and degradation. Both compounds exhibit potent cytotoxicity against 3Y1 cells.