The use of selective media is important in bioreactor problems. While the selective component of the medium (an antibiotic, for example) may be added from an external source, there seems to be an advantage from having the selective process generated internally. Models for two common ways of achieving selective media are considered. In the first, proposed originally by Sardonini and DiBiasio, the plasmid-free organism is auxotrophic for a metabolite which is produced by the plasmid bearing organism in excess. For this model we are able to characterize completely the global behavior of solutions, completing that theory. In the second, the plasmid-bearing organism devotes a portion of its resources to producing a toxin to the plasmid-free organism. Such a model was proposed by Chao and Levin and the model considered here is a slight variant of theirs. Again, the global asymptotic behavior of the model as a function of the parameters is obtained.