In place of conventional carrier bodies for attaching microorganisms used in various microbiological reactions in a bioreactor, the invention provides an improved carrier body, which is in a cylindrical, tubular, angular or spherical form having specified dimensions, formed of filaments of a synthetic resin having a specified diameter and intertwisted with each other to give a specified bulk density and specified void ratio. By virtue of the greatly increased capacity available for supporting microbial cells, the efficiency of various microbiological reactions can be greatly improved by using the inventive carrier for microorganisms in ethyl alcohol and citric acid fermentation, waste water treatment, etc. The carrier bodies of the invention can be used also in a fluidized-bed type bioreactor by either compressing the carrier body with a heated tool or covering the surface of the carrier body with a plastic-made net or perforated film to prevent intertwinement of the protruded filament ends on the surface of the carrier bodies.