The Interlake Group in Manitoba was examined in detail, both in outcrop and in the subsurface with correlations established between them. The outcrop area is described according to Stearn (1956) with some revisions suggested. The Fisher Branch Formation is not a recognizable unit and the Cross Lake Member of the Cedar Lake Formation is not distinctive. Several new sections at Grand Rapids and coreholes in the northern outcrop area contain thin sand and shale beds which can be correlated with extensive subsurface markers. In the subsurface, the Interlake Group has been subdivided by means of non-sequential marker beds, U, U2, and V, The Lower Interlake has been subdivided into the Strathclair and Brandon Formations. The use of the outcrop name, Cedar Lake Formation is extended to apply to the Middle Interlake. All formations are essentially dolomite, but through interpretation of relict textures, establishment of sedimentary pattern, notably reef distribution, has been attained. Some reefs have considerable dimension and affected sedimentation within the formation in which they occur and in some instances, in the overlying formation. Favourable sites for the accumulation of hydrocarbons are in porous zones and trapped at the pre-Middle Devonian unconformity.