Abstract The Lower Mississippian Alamogordo Member of the Lake Valley Formation in south-central New Mexico represents a well exposed carbonate-dominated homoclinal ramp with little evidence of sediment transport. Biotic components present and evident in thin section have distinct distributions in the level-bottom facies in relation to their shoreward and basinward ramp positions. Assemblages (I–IV) are established along the depth gradient from shallow to deep water, and are useful in palaeobathymetric interpretation; they are similar in composition to the depth-related phases recognized in Dinantian mounds by Lees & Miller (1985). The biotic assemblages shift laterally in successive beds of the Alamogordo Member, indicating variations in sea level during deposition. As a result, deepening and successive shallowing during a transgressive-regressive cycle can be tracked within the Alamogordo Member. The ramp slope is interpreted to have been of the order of 0.5°, based on the maximum depths for green and red algae having been 110m and 250m respectively. The systematic positions of several problematical taxa ( Asphaltinella, Sphaerinvia , stacheiins and salebrids) are discussed.