A new natural diketopiperazine (1) was obtained from the culture broth of Microbispora aerata strain imbas-11A, isolated from penguin excrements collected on the Antarctic Livingston Island. Compound 1 was purified consecutively by solvent extraction, silica gel column chromatography and preparative HPLC. The structure of the compound was elucidated by 1D and 2D NMR experiments and mass spectrometric investigations. The absolute configuration of compound 1 was determined by amino acid analysis and NOESY correlations. A low antiproliferative and cytotoxic effect of trans-cyclo-(D-tryptophanyl-L-tyrosyl) (1) was determined with L-929 mouse fibroblast cells, K-562 human leukemia cells and HeLa human cervix carcinoma. Trans-cyclo-(D-tryptophanyl-L-tyrosyl) (1) did not show antimicrobial activity at a concentration of 50 µg per disc against Bacillus subtilis, Staphylococcus aureus, Streptomyces viridochromogenes, Escherichia coli, Candida albicans and Mucor miehei.