78 strains of mycelial fungi were isolated from various soil samples of the Republic of Uzbekistan. A cup express method has been developed for the primary assessment of the acid-forming ability of mycelial fungi belonging to the genus Aspergillus and Penicillium. As a result of screening, two strains with the highest acid-forming ability were selected. It has been established that in natural strains in the dynamics of growth, the maximum concentration of citric acid (CA) biosynthesis is observed at the end of the lag phase or in the phase of exponential growth. In the growth dynamics of natural strains 5 and 8, the maximum biosynthesis of CA was observed within one or two hours, respectively.