This book was written as a partial fulfillment for the requirement of the degree of Master of Science in Zoology (Fisheries). The present fish production in Bangladesh is unable to cope up with the ever-increasing need of fish. Polyculture gives higher production than monoculture in extensive and semi-intensive systems because more of the available natural food in the pond is utilized by fish in polyculture. Periphyton based aquaculture systems offer the possibility of increasing both primary production and food availability for fish which can be practiced in Bangladesh, where there are huge untapped fisheries resources need to be developed for contributing to the economic growth of the country. This study was performed to compare the effects of addition of substrates and mola on production performance of mrigal and tilapia in polyculture systems. After successful completion of the experiment experiment it was found that the addition of substrates in polyculture resulted higher yield of fishes than the addition of mola. So we conclude that the periphyton based aquaculture technique can be an alternative of traditional fish culture practices in Bangladesh and around the globe.