Thin section studies of about 100 rock samples recovered from the Mid-Atlantic Ridge (FAMOUS area, near 37¿N) show that the occurrence of segregation vesicles is restrictd to particular textural zones corrsponding to the inner portion of the pillows. A detailedd study of an aphyric olivine basalt shows that the mineralogy of the segregated material is mainly clinopyroxene (salite or Ca-augite) and plagioclase (An 50 50 to An 60 intergrowths associated with titanomagnetite grains. Olivine and plagioclase microlites (An 65 to An 75) crystallized in the groundmass prior to the formation of the segregation vesicles. Scanning microprobe analyses of the segregated prodduct show an enrichment in Si, Fe, Ca, Na, Ti, P, and K and a depletion in Mg and Al with respect to the bulk rock composition. The compositional variatoin of the segregated material across the sample suggests that the degree of segregated melt differentiation increases toward the center of the pillow. The formation of segregation vesicles is a progressive phenomenon which occurs during a period of the cooling time when viscous deformation is still possible. This study provides criteria for recognizing portions or loose fragments of pillows which may be oriented by the method of Bideau et al. <1977>. This method is helpful for paleomagnetic studies of ocean floor basalt dredged samples.