Abstract: The process and apparatus according to the invention are suitable for sterile and/or partially sterile preparation, inoculation and bagging of mushroom substratum. The joint application of the process and apparatus enables the use of agricultural wastes and the production of substratum in reduced time so that the yield can be multipled compared with the practices known so far and the expenditures are lower than those known so far.In the course of the process according to the invention dry straw and/or corn-cob are comminuted to the required size and admitted together with additive into a closed system. The material is treated with 90.degree. C. hot water until 70% moisture content is reached in the first step, while it is continuously mixed, then it is sterilized by heating for 35 minutes. Next the material is cooled to 25.degree. C. and the internal pressure of the apparatus is equalized to atmospheric pressure.