Background: Poly-L-malic acid (PLMA) comprises aliphatic polyester polymers with broad applications in pharmaceutical industries. The fungal microorganisms are among the best natural sources recruited to supply L-malic acid (MA) as a precursor of PLMA. In this study, we investigated MA production ability of 7 clinical isolated of the fungus Aureobasidium pullulans. Materials and Methods: Seven clinical isolates of A. pullulans acquired from Westerdijk Fungal Biodiversity Institute were studied, and the isolate with the highest total MA production was selected for the optimization process. We tried to optimize the output by applying different concentrations of CaCO3 in fungus medium (1.5%, 3%, and 6%) and various incubation temperatures (27°C, 32°C, and 37°C) during 3, 7, and 14 days. Results: Intra-strains variation was significantly strong (P<0.0001), and the highest production of MA was carried out by the isolate A. pullulans var. melanigenum dH 21931, UTHSC 06-456. The amount of MA produced by this strain was significantly higher in medium with 3% CaCO3 compared with other concentrations of CaCO3 and after 7 days incubation than the other fermentation times (P<0.05). Although MA production was higher at 27°C, the differences between the investigated various temperatures were not significant (P>0.05). Conclusion: Overall, we obtained the highest MA production in Sabouraud dextrose agar (SDA)medium with 3% CaCO3 at 27°C after 7 days of incubation. Our study indicated that the fermentation period and CaCO3 concentration significantly alter MA production in A. pullulans var. melanigenum.