Cyclic peptides containing non-proteinogenic amino acids often exhibit a broad bioactivity spectrum and many have entered clinical trials with good prospects for drug development. We recently reported the discovery of six cyclic tetrapeptides, the pseudoxylallemycins A-F (1-6), from a termite-associated Pseudoxylaria sp. X802. These compounds contain a rare O-homoallenyl-L-tyrosine moiety and showed promising antimicrobial activity against the Gram-negative pathogenic bacterium Pseudomonas aeruginosa. To perform more detailed structure-activity studies, we pursued a precursor-directed diversification strategy. Here, we report the purification, identification and testing of 21 new pseudoxylallemycin derivatives.