An unusual ultramafic rock characterized by elongate olivine crystals forms several small masses in alpine-type, tectonitic peridotite of the Preston Peak ophiolite, Klamath Mountains, California. The largest mass of this distinctive ultramafic rock, which has been mapped in detail, has contacts with the surrounding peridotite tectonite that vary from indistinct to sharp. In several areas, the country-rock tectonitic peridotite is impregnated by a network of talc-olivine veins locally containing elongate olivine crystals. Within the mass, the elongate olivine crystals, many of them pseudomorphosed by serpentine, are typically arranged in a crisscrossed pattern (jackstraw texture), but radial and parallel patterns also occur. The olivine crystals are bladelike but lack the skeletal or dendritic textures characteristic of elongate igneous olivines. Microprobe analyses of the olivine blades indicate that they are magnesium-rich (Fo89.4) and unzoned. The coexisting mineral assemblage includes talc, tremolite, magnesite, chlorite, and pentlandite, minerals that are not compatible with a magmatic origin. Furthermore, at several localities within the mass, relict structures and minerals of the peridotite tectonite are preserved in the Jackstraw-textured rocks, indicating a replacement origin. Major- and minor-element chemical data support a replacement origin, in that the Jackstraw-textured talc-olivine rocks are similar in chemical composition to the surrounding tectonitic peridotite of the ophiolite. The most obvious difference is that the jackstraw-textured rocks contain more sulfur. Experimental data on the coexisting mineral assemblages indicate a low-temperature (500 to 600 °C) origin, and field relations suggest the importance of a volatile-rich phase.