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ESR measurements of KH2PO4 crystals doped with arsenic exhibit altered line shapes similar to those produced by an incommensurate phase. The modified line shapes occur in a range of temperatures extending approximately 17 K below the paraelectric-ferroelectric critical temperature T, . The form and temperature dependence of the lines are in reasonable agreement with the predictions of the phasesoliton theory. The parameters h & and h2 determining the variation of the resonant magnetic field show a power-law behavior as a function of temperature. The significance of the experimental results is discussed in the light of earlier studies which indicate a ferroelectric structure with four types of domains differing in electric polarization and in spatial orientation.Experimental evidence in the form of altered electronspin-resonance line shapes indicates strong distortion in crystals of KH2PO4 doped with arsenic in a temperature range extending from the ferroelectric transition temperature 123 K to approximately 106 K. The form and ternperature dependence of the line shape in this range are consistent with the existence of an incommensurate phase. The data, in fact, agree with the predictions of the one-dimensional phase-soliton model, and the temperature dependence of the parameters is consistent both with theory and with observations in other ferreoelectric systems. There is, however, no direct evidence to indicate that an incornrnensurate phase exists in this material, and an alternative interpretation in terms of the distortions associated with domain walls will also be considered. Since no quantitative predictions following from this second interpretation are available, and since certain features discussed below make it problematical, we present a comparison of our results with the soliton model of an incommensurate phase even in the absence of corroborating evidence or a clear picture of the structure of such a phase.A number of ferroelectric crystals' exhibit phases in which the ions of the lattice have a modulated displacement with a period that is incommensurate with the period of the lattice. Most of these are ferroelectrics of the displacive type, although a few do not show a soft mode above the temperature TI below which an incommensurate phase appears. These crystals become ferroelectric at a temperature T, ( ( TI ) at which the periodicity of the modulated displacement, which is correlated with the electric polarization, "locks in" to a value commensurate with the lattice constant. In this paper, we present experimental evidence for the existence of distortions of the lattice similar to those produced by an incommensurate phase in a doped ferroelectric from a family of the order-disorder type. In the doped potassium dihydrogen phosphate (KDP) crystals examined in this experiment, the evidence indicates the onset of distortion at or near the ferroelectric-paraelectric critical temperature T, =123 K. This distortion persists to a lower temperature of about 106 K, where the data indicate the normal line sha...