Head and body length of S. bilineata is 47-56mm with tail lengths of 16-23mm. Weights range from 6-9g. The upperparts are blackish with two prominent, buff colored stripes extending from the neck to the rump. The underparts are dark gray. The wing sacs are well-developed in males and open along the forward edge of the upper side of the forearm. Wing membranes are blackish and hairless. Females are slightly larger than the males.

S. bilineata roosts in tree holes, buttress cavities, and on the walls of buildings. They generally roost in groups made up of several harems of 5-50 individuals. Each male defends between 1-9 females, using strong smelling secretions from the wing sacs with which they "salt" their females. Territorial displays and audible songs are also used to maintain the harem. These behaviors are often observed near roost sites at dusk or dawn.

Shortly before sunset, individuals leave the roost to forage for insects in the lower levels of the rainforest, later moving farther afield where the insects are more plentiful.

One young is born annually, at the onset of the rainy season. Females carry their young to individual hiding sites when foraging. Young can fly at 2 weeks, but continue to suckle for several months.

They are distributed in Jalisco and Veracruz (Mexico) to Bolivia, Guianas, and E Brazil south to Rio de Janiero; Trinidad and Tobago.