Using inexpensive sources is a crucial point in the production of valuable compounds such as lactic acid. In the present study, the production of lactic acid by an efficient isolate of S. thermophilus was optimized using whey as a carbon source and yeast extract as a nitrogen source. MRS culture medium with 6.5 % NaCl was used for the isolation of streptococci from industrial yogurt samples and the selected isolate was identified using the 16S rRNA gene sequencing. Evaluation of lactic acid production was done by the Randox method. Lactic acid production by the selected isolate was optimized by response surface methodology (RSM) in MRS broth regarding to different concentrations of whey and yeast extract. The isolate that produced the highest amount (8.9 g.L-1) of lactic acid within 52 h growth in MRS broth was identified as a strain of S. thermophilus by molecular identification. Optimization of concentrations of carbon and nitrogen sources resulted in the induction of lactic acid production by Streptococcus thermophilus up to 24.18 g.L-1 in the presence of 3.5 % of whey and 5 % of yeast extract as external carbon and nitrogen sources in the MRS medium, which was similar to the value predicted by RSM. Yeast extract was found to be more effective on lactic acid production than whey. Optimization of lactic acid production by low-cost substrates whey and yeast extract resulted in high induction of lactic acid production by S. thermophilus compared to some other studies that used other cost-effective substrates and/or bacteria isolated from traditional dairy products.