The Montgary pegmatite is located at Bernic Lake in the southeastern Manitoba pegmatite district. It has a complex internal structure with several contrasting mineral assemblages. The pegmatite contains a number of potentially valuable minerals - including spodumene, amblygonite, lepidolite, zinnwaldite, pollucite, and beryl. The distribution of each of these within the pegmatite is related to the internal structure and controlled by a specific mineral assemblage. Parts of the pegmatite contain a high concentration of spodumene. The pollucite occurrence is of unique character and is one of the largest known deposits of this mineral. The pegmatite is sill-like in attitude with a maximum dip of about 15 degrees . Its contacts with the amphibolitic wall rocks are sharp and tourmalinized. Mineral assemblages present include perthite-plagioclase-quartz-muscovite, spodumene-microcline-perthite-plagioclase-quartz-muscovite, cleavelandite-lithia micas-quartz, quartz amblygonite, albite, lepidolite, pollucite, and quartz assemblages. The occurrence and relationships of these are described, and possible relationships between attitude and internal structure are discussed. Many features of the pegmatite suggest that the deposit formed by crystallization and differentiation of a pegmatite fluid in a restricted system. These features are discussed. The nature and occurrence of the pollucite, lepidolite, and albite assemblages are such, however, that it is difficult to visualize them resulting from such a system. These assemblages may be of later replacement origin. The source of the late, alkali-rich replacing solutions may be unmixing of soda and the rarer alkalis (Li, Rb, Cs) from earlier-formed silicates in a manner analogous to the unmixing of soda feldspar in perthites. Various aspects of this hypothesis are examined.