Based on the Nash equilibrium evolving in an extensive-form, not a normal-form game, it is found that virulence reduction of ?, and skill improvement of lambda coli is a reasonable outcome of direct temperature induction of bacteriophage lambdaQ(-)-mutant in Escherichia coli. This cooperative situation evolved as the Nash equilibrium is basically considered as a long-term sustainable development strategy for species coexistence. A host demands a lambda-free environment by skill improvement for normal growth. Meanwhile, lambda chooses a less virulent strategy to maintain host viability for the sake of its survival. Virulence reduction of lambda enhances the coexistence of lambda and E coli; thus, moderate induction results. This strategic game obviously clarifies certain hypothesized results shown in previous works.