Using Brewster angle microscopy to study fatty acid Langmuir monolayers, Overbeck and Mobius (J. Phys. Chem. 1993, 97, 7 999)have observed changes of texture indicating a new phase boundary not visible in monolayer isotherms. We have studied Langmuir monolayers of CH3(CH2)(17)COOH at 30 degrees C using X-ray diffraction. We find that the monolayer undergoes a first-order phase transition from the known L(2) phase in which molecules tilt toward a nearest neighbor to a phase with tilt toward a next-nearest neighbor. Both before and after the transition, the structures are hexagonal in the plane perpendicular to the molecules. Neither the intermolecular spacing in this plane (approximate to 4.85 Angstrom, close to that seen in the Rotator II phase) nor the tilt magnitude has an observable discontinuity at this transition; only the tilt direction changes. As a result, the isotherm area/molecule is also continuous across the transition.