The eigenmode structure of drift waves in a straight stellarator using the ballooning mode formalism is investigated. The electrons are assumed to be adiabatic and the ions constitute a cold, magnetized fluid. The ‘‘effective potential’’ has an overall parabolic envelope but is modulated strongly by helical ripples alongB. Two classes of solutions are found: those that are strongly localized in local helical wells, and those that are weakly localized and have broad spatial extent. The weakly localized modes decay spatially due to the existence of Mathieu resonances between the periods of the eigenfunction and the modulation of the ‘‘effective potential.’’