To clarify the internal fatigue crack generation in a Ti-Fe-O (near ␣-type) alloy, microstructures, internal fatigue crack initiation sites, and dislocation structures in samples fractured during high-cycle fatigue tests at liquid nitrogen temperature were studied. The alloy contained two kinds of elongated ␣-phase microstructures, i.e., recovered ␣ grains and recrystallized ␣ grains. Untested samples contained mobile dislocations in recovered ␣ grains, but in recrystallized ␣ grains, any dislocations were observed. Internal crack initiation sites were formed transgranularly and were related to the recrystallized ␣ grain region, judging from their morphology, size, and chemistry. Dislocations in recovered ␣ grains were rearranged after cyclic loading in either {0110} Ϫ ͗1120͘ planar arrays or subgrain structures due to dislocation annihilation. Few dislocations were seen in recrystallized ␣ grains. We discuss the relationship between localized strain incompatibility due to coplanar arrays in recovered ␣ grains and transgranular cracking in recrystallized ␣ grains, and propose a model for fatigue crack generation. H. YOKOYAMA, formerly Graduate Student, Department of Mechanical atomic size misfit, solute content, short-range order, and Engineering, Kogakuin University, is Engineer, Nittan Valve Co. Ltd., dispersion particles. [16,17] Thus, internal crack initiation must Kanagawa 257-0031, Japan. O. UMEZAWA, Senior Scientist, and K. be related to the size of the initiation site, [18] applied stress, [10] NAGAI, Unit Leader, are with the Frontier Research Center for Structural and microstructural factors [5,11,18,19] such as grain size, distri-Materials, National Research Institute for Metals, Ibaraki 305-0047, Japan. T. SUZUKI, Emeritus Professor, and K. KOKUBO, Professor, are with the bution, chemistry, and texture. In studies of internal crack Department of Mechanical Engineering, Kogakuin University, Tokyo 163initiation for a Ti-5Al-2.5Sn extra-low interstitials (ELI) 8677, Japan. alloy, [20] localized slip in a specific inhomogeneous micro