The cyanobacterium Tychonema sp. produces the new cyclic hexapeptides brunsvicamide A-C (1-3). Brunsvicamide B (2) and C (3) selectively inhibit the Mycobacterium tuberculosis protein tyrosine phosphatase B (MptpB), a potential drug target for tuberculosis therapy for which no inhibitors are known to date. Brunsvicamide C contains an N-methylated N'-formylkynurenine moiety, a unique structural motif in cyclic peptides. The new peptides are related to the sponge-derived mozamides, supporting the suggestion that secondary metabolites of certain marine invertebrates are produced by associated microorganisms. Thus, microorganisms phylogenetically related to symbionts of marine invertebrates can be judged as a means to supply "marine-like" compounds for drug development.