Abstract This paper describes, in considerable detail, how a mechanistic mathematical model can be constructed to express the activity of an organism of importance to food microbiology. In this demonstration model, a metabolic marker of Clostridium sporogenes (acetic acid production) was the response variable and the environmental variables were medium composition, incubation time and incubation temperature. To construct a mechanistic model, mechanical precautions were observed to minimize experimental and machine errors, and the physiology of the organism is discussed. If those precautions were not observed and the physiology not understood, the result would have been an exercise in curve fitting. The statistical and mathematical procedures used are readily available from commercial sources, and the final equation is simple enough to be programmed into a hand-held calculator. Suggestions were made for the exploitation of this approach to other problems in food microbiology.