Zhijiang Baijiu is a Chinese strong flavour liquor produced using a traditional Chinese solid state fermentation involving microorganisms in pit mud on cellar walls. In this study, pit mud samples were collected from three different cellars in a Zhijiang Baijiu distillery ranging in age from 10 to 20 and 30 years. The bacterial communities were analysed using denaturing gradient gel electrophoresis, high‐throughput sequencing and quantitative real time PCR. These analyses showed that the diversity of the bacterial community in pit mud samples was relatively stable in cellars aged from 20 to 30 years old. High throughput sequencing also revealed that the relative abundance of seven core bacterial families and nearly half of the core bacterial genera were stable in 20 year‐old cellars. In cellars of 10 to 30 years, the relative abundance in pit mud of Ruminococcaceae and Clostridium cluster IV increased, while that of Petrimonas and an unclassified Firmicutes decreased. Real time PCR showed that aged pit mud contained more bacteria and Clostridium kluyveri than young pit mud. It is concluded that the bacterial communities in aged pit mud are more suitable for producing high quality Baijiu. © 2019 The Institute of Brewing & Distilling