A computer simulation model was developed which describes growth and competition of bacteria in the soil environment. In the model, soil was assumed to contain millions of pores of a few different size classes. An introduced bacterial strain, e.g. a genetically modified micro-organism (GEMMO), was assumed to distribute among the pores of the different size classes. This strain (the invader) was assumed to compete with the indigenous soil biota. Key aspect in this competition was the available pore space. The model predicted that generally the GEMMO invader bacteria ultimately did not become established in soil. Extinction of the invader was, however, often slow, and could take in the order of years. Very harsh conditions in soil were seen to enhance the pore space available for colonization and hence the probability that the invader was able to expel the indigenous microflora and become established. Extinction of the invader was slower when it was introduced when soil conditions were becoming more favourable for microbial life than when conditions were worsening.