The electrostatic precipitator (ESP) is usually employed by industries for the removal of aerosol particles from gaseous streams. The behavior of the particles inside the collection duct is of great interest as a tool for optimizing precipitator design and performance. In this work, a laboratory scale ESP (0.6m long, 0.3m high and 0.2m wide) was tested with electric fields of 250, 350 and 450 kV/m at a gas velocity of 1.0 m/s. The discharge electrode was located at 0.30 m from the beginning of the collector plate. The test aerosol was constituted of a phosphatic concentrate dispersed in air. Particles were sampled isokinetically in situ in the duct, at five different longitudinal positions. The experimental results for the aerosol studied show that concentration profile of particles are affected by the electrical field at considerable upstream distances from the charging electrode. The particle collection starts well before the charging electrode, characterizing a strong entrance effect, often disregarded in the existing correlations for predicting ESP performance.