Cellulose and xylan are the most common Non-Starch Polysaccharides (NSPs); they are available in plants and they exhibitanti-nutritional effects. The present study is intended to detect cellulose and xylan degrading autochthonous gut bacteria inIndian major carps (Labeo rohita, Catla catla, and Cirrhinus mrigala) and exotic carps (Hypophthalmichthys molitrix,Ctenopharyngodon idella, and Cyprinus carpio); it is also meant to identify the most promising strains by molecularmethods. The promising strains were also tested for likely antagonism against few pathogenic Aeromonas strains.Altogether, 432 microbial strains were isolated on media containing either cellulose or xylan. Seventy strains wereprimarily selected through qualitative enzyme assay. Finally, the quantitative assay led to the selection of 5 promisingNSP-degrading strains (LFR1X, CMF1C, HMF6X, CtIF1C, and CMH8X). Amongst these, LRF1X was the best cellulaseandxylanase-producer. Analyses of 16S rRNA partial gene sequence revealed that strains LRF1X and CMF1C wereclosely related to Bacillus pumilus (Accession numbers; KF640221, KF640223, respectively), whereas HMF6X, CtIF1C,and CMH8X were similar to B. tequilensis (KF640219), B. megaterium (KF640220) and B. altitudinis (KF640222),respectively. The culture of the selected microorganisms with autochthonous bacteria and yeasts indicated their coexistencewithin the fish gut. An appraisal of antagonism against four pathogenic Aeromonas species by the cross-streakingmethod revealed that the selected NSP-degrading strains (except CMH8X) were antagonistic to at least 2 pathogens. Invivo bio-safety assessment through intra-peritoneal injection of the isolates showed no induction of pathological lesions ormortality in healthy laboratory acclimatized rohu, L. rohita.