The mode localization phenomena in bottom-hole assemblies (BHA) used in oil-well drilling is investigated A fully gaged stabilizer model is shown to produce weak coupling between segments of drillcollars separated by stabilizers. It is observed that in a weakly coupled BHA, small disorder in span length can cause strong mode localization. For the first time, it is demonstrated that gravity induces a stiffness disorder which in turn causes strong mode localization in an inclined wellbore, even though no other disorder is present The effect of localized modes for failure is presented by examining the maximum bending stresses with and without mode localization. The results show that design and operation of drillstrings should consider the possibility of mode localization for an accurate prediction of dynamic behavior.