Aims: To determine the susceptibility of cyanophycin granule polypeptide (CGP) to degradation by several mammalian, avian and fish gut flora. Methods and Results: Samples of gut flora were investigated for the occurrence of bacteria capable of CGP degradation. With all samples, a complete anaerobic degradation of CGP was achieved over incubation periods of only 12-48 h at 37 degrees C. CGP-degrading bacteria were detected in all samples, and they occurred in particular high titres in caecum flora from rabbit and sheep and in the digestive tract of carp fish. A total of 62 axenic cultures were isolated. All degraded CGP aerobically, 46 of them degraded CGP also anaerobically over incubation periods ranging from 24 h to 7 days. HPLC analysis revealed that all isolates degraded CGP to its constituting dipeptides. Eight strains were identified by 16S rRNA gene sequencing and were affiliated to the genera Bacillus, Brevibacillus, Pseudomonas, Streptomyces and Micromonospora. Conclusions: These data demonstrate for the first time the occurrence of a natural niche for CGP in the digestive tracts of animals. Significance and Impact of the Study: The biodegradability of CGP by gut flora provides a first confirmation for the potential applications of CGP and its dipeptides in nutrition and therapy as highly bio-available sources for arginine, lysine, aspartate and possibly also other amino acids.