Selection pressure is widely used in stabilizing the plasmid of the recombinant microorganism. The effects of selection pressure on enzyme production were investigated using recombinant E. coli harboring plasmid pTA322. Optimum ampicillin concentration which maximizes the enzyme production was 150㎍/㎖ in flask culture and 50㎍/㎖ in the culture using 2-1 fermentor. Optimum concentration of selection pressure in a stirred fermentor was different from that in a flask due to different agitation effects. It was found from mathematical modelling that as the concentration of ampicillin increased, the plasmid-harboring cells with low plasmid copy number were damaged severely, while the specific production rate increased due to the high plasmid copy number in the microorganism.