The Mermentau River drainage in Louisiana has been largely ignored by turtle biologists, despite historical records of its turtle species. In particular, almost nothing is known about the range and abundance of the Sabine map turtle (Graptemys sabinensis) in the Mermentau drainage, which is frequently omitted from descriptions of the species' range. Four specimens of G. sabinensis were collected from the upper Mermentau in 1893 and 1894, but no trapping of the species has been conducted in the drainage since. We determined basking density and relative abundance among basking turtles for Sabine map turtles in the Mermentau drainage, including the Mermentau, Lake Arthur, and 5 major tributaries. We recorded turtles seen during boat surveys and in point-counts from bridges and other access points. The Mermentau and Bayou Plaquemine Brule had the highest basking densities, ranging from 14.0 to 34.5 G. sabinensis per river kilometer. Bayous des Cannes and Nezpique and Lake Arthur had intermediate average densities of 5.0–9.7 G. sabinensis per river kilometer and the river below Lake Arthur and the southern tributaries, Bayou Queue de Tortue and Lacassine Bayou, had the lowest average basking densities, ranging from 0.7 to 1.4 G. sabinensis per river kilometer. Sabine map turtles were the most abundant basking turtle on the Mermentau drainage, accounting for 73% of all turtles seen. Graptemys pseudogeographica, the false map turtle, accounted for only 0.2% of all turtles observed and is approaching extirpation in the drainage, likely due to the near total eradication of mussels that has resulted from extensive dredging of the Mermentau and its major tributary bayous.