The distribution and ecology of the lancelet Branchiostoma platae (Hubbs, 1922) in the Ilha Grande (RJ) region are studied. Hydrological differences between the two regions of Ilha Grande (RJ) are reinforced by the distribution of this species. Its largest density reaches 717 individuals/m². Most of the occurrences was in coarse sand bottoms. Positive correlation between water mass, currents and the occurrence of the species is suggested. There is a positive correlation between the lenght mean/station of the body with the granulometry mean of sediment. An accentuate annual variation in the density of the population in some places of the region is verified.