A living, linear reef about 3.8 km in length has been discovered 0.8 to 1.2 km off Big Pine Key and the Newfound Harbor Keys in the lower Florida Keys. The reef is approximately 60 m wide, lies at depths from 4.5 to 7 m, and is growing on, and slightly landward of, a slope where Pleistocene bedrock rises with relative abruptness. A deeper, flat area on the seaward side of the reef is, in most places, a Thalassia-Cymodocea (sea grasses) meadow which is usually separated from the reef by a narrow, barren belt of fine-grained sediment. The outer portion of the reef consists of small, scattered coral heads and octocorals separated by a patchy veneer of sediment. The main body of the reef includes large, commonly coalescent coral heads, some standing at least 2.5 m above the sea floor. Next landward is a zone of smaller coral heads and abundant octocorals, followed by an area of large heads of Montastrea annularis. The previously known patch reefs off the Newfound Harbor Keys lie farther landward in shallower water. Although the sediment veneer is thin across much of the linear reef, accumulations of skeletal sand and gravel up to 50 cm thick, are common in areas of greatest coral growth. Geometry and biota of this reef, as well as its location subparallel to the present shoreline, suggest that the reef may be a living analog of at least a part of the Pleistocene Key Largo Limestone.