Aims:  To evaluate cellulase production by Streptomyces malaysiensis in submerged fermentation using brewer’s spent grain (BSG) and wheat bran (WB) as carbon source, and corn steep liquor (CSL) as nitrogen source, as compared to yeast extract (YE), and partial characterization of the crude enzyme. Methods and Results:  Maximum cellulase production by Streptomyces malaysiensis (720 U l−1) occurred within 4 days incubation when using a growth medium containing BSG 0·5% (w/v) and CSL1·2% (w/v). CMCases activity showed to be stable over an acidic pH range (2·0–7·0) and in temperatures of 40–60°C. Zymogram indicated three bands of CMCase activity, with different molecular masses. Conclusion:  S. malaysiensis was able to grow and produce good levels of CMCases using solely brewer’s spent grain and corn steep liquor as low‐cost substrates, making this strain and these low cost by‐product worthy for further investigation, and potentially feasible for biotechnological applications in different areas. Significance and Impact of the Study:  To our knowledge, this is the first study reporting the use of the low‐cost by‐products brewer’s spent grain and corn steep liquor, as sole substrates for microbial enzyme production.