Experiment was performed to ascertain the effect of fish stocking density on population growth performance of some biogeochemical cycling bacteria in carp culture system. Advanced fry of Catla catla L., Labeo bata Ham. and Cyprinus carpio L. were introduced into tanks @ 8, 13, 16 and 32 fry tank-1 maintaining the ratio of C. catla -2 : L. bata -3 : C. carpio -3. Each tank was fertilized with mixed fertilizer @ 200 g week-1 during the rearing period of 150 days. Water samples were collected for examination of bacterial population, primary productivity and water quality parameters at weekly and biweekly intervals. Growth of fish was recorded at regular intervals. Statistical analysis of collected data was done by ANOVA and LSD for separation of mean (P < 0.05). Bacterial growth rate and productivity tended to rise with increasing stocking density showing maximum at stocking density 32 fry tank-1 whereas, fish growth exhibited an increasing trend upto stocking density 16 fry tank-1 and declined in further increment of fish density. Stocking density dependent growth efficiency of different bacteria (heterotrophic bacteria, cellulose decomposing bacteria, denitrifying bacteria and phosphate solubilizing bacteria) varied from 18 to 62.7%, 45 to 140%, 61 to 238% and 79 to 382% in stocking density 8, 13, 16 and 32 fry tank-1 , respectively. Results clearly revealed that higher stocking density of fishes liberates higher quantity of faecal matter containing proportionately greater amount of nutrients having more scope for bacteria to act upon the faecal matter resulting in higher bacterial population and productivity in one hand and fish growth inhibition due to stressful and uncongenial environmental condition developed by means of heavy nutrient load on the other hand. Therefore, it may be concluded that stocking density influencing the growth of biogeochemical cycling bacterial population in fish culture system.