Crystals of authigenic, euhedral tourmaline are present in a core from a productive shallow gas reservoir in the Tiger Ridge field, Montana. Overgrowths on detrital tourmaline grains have been reported by many authors, but only one report is known of discrete euhedral crystals, and these were interpreted as having a detrital origin. The tourmaline crystals discussed in this report are clearly authigenic. The tourmaline occurs as acicular or prismatic crystals, 10 to 120 microns in length, usually in intergranular pore spaces. The crystals display distinctive optical properties and typical tourmaline morphological development. Their formation appears to be controlled by access to larger pore spaces. The Upper Cretaceous Eagle Sandstone, in which the tourmaline occurs, displays abundant evidence of infrastratal alteration, and the tourmaline crystals are often associated with authigenic quartz and clays, Detrital grains of tourmaline are also present, and most display evidence of authigenic overgrowths similar to the euhedral crystals. The authigenic tourmalines probably exert several negative effects upon reservoir quality. Waters associated with Bearpaw Mountains volcanism are a likely causative agent in the formation of the authigenic crystals. This hypothesis apparently presents several constraints involving the timing of tourmalinization and gas emplacement in the Tiger Ridge field.