Once found in Pierce County, Washington and from San Francisco Bay to the San Bernardino Valley in California, marsh sandwort today is known from fewer than three localities, and its numbers have dwindled to perhaps less than perhaps 50 individuals. As early as 1915, marsh sandwort was considered to be one the rarest plants in Washington, and throughout the first half of 20th Century, botanists stated that it had been seldom or rarely collected elsewhere (Abrams and Ferris 1944; Anonymous 2008a; Maguire 1951; Piper and Beattie, 1915; Wiegand, 1897). The only known locality in San Francisco Bay was near Fort Point (under the Golden Gate Bridge), where it was reported to be very abundant in swamps (Kellogg 1863). It has not been seen there since 1899. The last collection made in San Bernardino County was in 1899; the last observation at a single location in the Santa Cruz Mountains was in 1976. Today, marsh sandwort is known from as few as two localities in southern San Luis Obispo County. At one of these localities, it co-occurs with Gambels watercress, another endangered species. Marsh sandwort has been reported from Mexico and Guatemala, but its distribution there also appears to be highly restricted (Bonilla 1992; Hartman et al. 2005; Islebe 2003). Marsh sandwort is a delicate herbaceous perennial, producing mat-like clusters of erect, slender shoots from underground runners (Abrams and Ferris 1944; Hartman et al. 2005; Hitchcock 1964; Mason 1957). The vegetative shoots often twine around each other or sprawl over associated vegetation and have been reported to reach up to 1 meter in length. Shoots typically have many pairs of opposite, linear leaves, with blades 1-2 cm long. The flowering shoots, which produce small, solitary, white flowers, often ascend among the leaves and stems of other marsh plants like rushes and sedges. Flowers are about 8-10 mm wide at anthesis, bear 5 white petals, and 10 stamens. Flowering is sporadic, ranging from May through August. Each flower produces a few, small, black seeds, but the type of potential pollinator remains unknown (Mazer and Waddell 1994; Mazer 2000).