Although Bothrops insularis is critically endangered, there is no estimate of its population size based on field quantifications. Here we provide the first estimate of its population size based on plot surveys conducted at the Queimada Grande Island. We distributed 26 quadrat plots regularly in a trail that crosses the island. Plots were sampled twice in 2002. Of the 26 plots we sampled, 21 were located in forests and five in grassy areas. For the first survey, mean estimates of population size for plots located in forests and grassy areas were 2134.3 and 224.0 snakes, and the Percentage Relative Precision for these estimates were 38.6% and 277.6%, respectively. For the second survey, no snakes were found in grassy areas, whereas the mean estimate of population size for plots located in forests was 1304.3 snakes and the Percentage Relative Precision for these estimates was 93.7%. Although suffering from relatively low precision, our best estimate of population size in B. insularis is around the lower end of the guesses found in the literature (2000–4000 individuals for the entire island). Furthermore, based on encounter rates obtained in a non-systematic way in the last 12 years, we have the impression that the density of B. insularis decreased in this period and we have evidence for the illegal removal of snakes from the island in the last few years. Our results suggest the urgent need of enforcement to restrain the illegal removal of snakes from the island and of a monitoring program to track future changes in the population size of B. insularis.