Exciton polaritons are considered in infinite multiple quantum wells which are periodic except that one well (impurity) has a different well width from the others (host). Under certain circumstances it is possible to have a long‐lived exciton‐polariton mode localized about the impurity. From the viewpoint of the electromagnetic field, this corresponds to waveguide modes in the vicinity of an exciton resonance in such structures. ©1995 American Institute of Physics.