We conducted Least Bittern (Ixobrychus exilis) surveys on Reelfoot Lake and nearby Black Bayou Waterfowl Refuge, in northwest Tennessee, USA during May–June 2003 to determine the distribution of Least Bitterns among structurally different vegetation types, including giant cutgrass (Zizaniopsis miliacea), swamp loosestrife (Decodon verticillatus), and woody vegetation. Least Bitterns were historically abundant on Reelfoot Lake when giant cutgrass once occupied 1,000 ha, but water-level stabilization resulted in 93% of the emergent zone now being dominated by swamp loosestrife interspersed with trees and saplings, including baldcypress (Taxodium distichum) and maple (Acer spp.). Least Bitterns were detected 49 times. Sites with Least Bitterns had greater percent giant cutgrass coverage, less woody vegetation coverage, and fewer tall trees than sites where Least Bitterns were not detected; however, Least Bittern presence was positively related with only giant cutgrass coverage and was unrelated to woody vegetation coverage or tall tree density. Density of Least Bitterns was greater on Black Bayou (0.45/ha) than Reelfoot Lake (0.11/ha). The lower density of Least Bitterns on Reelfoot Lake is partially attributed to the shift in the dominant emergent species from a grass to a shrub. In the absence of vegetation management, Least Bitterns will continue to decline on Reelfoot Lake as many swamp loosestrife marshes convert to forests.