ABSTRACT.  Cyperus louisianensis Thieret is an obscure member of subgenus Pycreus.  It was first described in 1977 and was previously thought to be endemic to two sites in southeastern Louisiana.  Cyperus louisianensis was listed by the United States Fish and Wildlife Service in category 2 among endangered or threatened species.  Since 1993, intensive field surveys have revealed more than 30 populations in southeastern Louisiana and southern Mississippi and isolated populations in southern Alabama and southeastern Georgia.  Field observations show that C. louisianensis is often locally abundant and weedy, and an analysis of specimens of it and the Old World weed, C. sanguinolentus Vahl, indicates that the two are conspecific.  Data on the distribution and ecology of C. louisianensis will be presented, and its taxonomic relationship to C. sanguinolentus and to other species of subgenus Pycreus occurring in eastern North America will be discussed.