Populations of the bacterium Escherichia coli and of its phage lambda vir appeared to equilibrate in continuous cultures. The bacterial end-populations were heterogeneous in respect of their resistance to lambda vir and their ability to utilize maltose. The most competitive of the selected bacteria were mutants which had a reduced rate of synthesis of lambda-receptor so as to become highly, but not totally, resistant to the phage. The coexisting phage had an increased affinity for the receptor and an altered antigenic specificity, suggesting adaptation of its adsorption site in response to the evolution of resistance in the bacteria.