Here we present the results of a population survey of the black-faced lion tamarin, Leontopithecus caissara, on Superagüi Island, and we extrapolate this estimation to the entire population. We used the linear transect method for surveys of the area from November 2000 to January 2002. When lion tamarins were seen in transects, we noted the perpendicular distance from the path and the number of individuals sighted in a cluster. Using the software Distance 4.0 we estimated a density of 1.79 individuals/km² and a population of 205 individuals (95% CI: 79 – 537 ind/km²) on the Superagüi Island. The extrapolation of these results to populations on the mainland gave us an estimate of 187 individuals (95% CI: 72 – 489 ind/km²). Our estimate for the total population is 392 individuals (95% CI: 218 – 1,026 ind/km²). The lower population density of L. caissara compared to other species of the genus, in combination with its very small geographic range, indicate that it is critically endangered. We also discuss some aspects of the limitations of the linear transect method when densities are extremely low. The Distance 4.0 software suggested a minimum of 30 sightings for a density estimate; but from the sighting frequency in our study this would require 2,489 hours and 883 surveys, not logistically viable.