The rare maritime ringlet butterfly (Coenonympha tullia nipisiquit McDonnough), which inhabits salt marshes in maritime Canada, experiences tidal flooding throughout its development. Its low vagility and the imminence of sea level rise call for the immediate investigation on the effect of flooding on survival. The maritime ringlet’s tolerance to tidewater submergence for up to 24 h was compared with that of the inornate ringlet (C. tullia inornata Edwards), a subspecies occupying nearby upland meadows. The suitability of salt meadow cordgrass [Spartina patens (Aiton) Muhl.], a major host of the maritime ringlet, for neonate inornate ringlet larvae was tested as well. Maritime ringlet larvae showed higher tolerance to tidewater submergence than inornate ringlet larvae, which were more likely to die from submergence stress after a few days. Although tidal submergence is probably not lethal to the maritime ringlet, there is a possibility of negative impact caused by prolonged submergence during storm tides. The inornate ringlet larvae failed to adopt S. patens, and all larvae died within 1 wk when forced to feed on it. The results suggest that the maritime ringlet has evolved physiological adaptations to saline wetland conditions that have enabled it to exploit its current habitat.