Aims: To analyse the sensitivity of a microbial culture to variations in the cultivation conditions by using the motile intensity of the cells. Methods and Results: Batch cultures of Bacillus thuringiensis were used to study the sensitivity of the cells to pulse changes in pH, temperature and oxygen supply. A droplet of the culture sample was visualized under an optical microscope and the image of the moving cells was captured with a computer controlled display camera attached to the microscope. Motile intensity was computed directly using an image analysis programme. The results showed that the different phases of cell growth exhibited different motile intensities. The motile intensity changed remarkably at the high level of the motile intensity, when the environmental variables are changed. Conclusions: The product formation was considerably reduced when a disturbance was applied at the high magnitude of motile intensity. Significance and Impact of the Study: Monitoring the motile intensity by image analysis is simple and makes it an attractive method for assessing the effect of environmental variables on the growth and product formation of microbial cultures.