The dilution method, devised by Petrobras (Brazilian State Oil Company), is the first initiative in Brazil to tackle ballast water management. This method is based on loading the ballast water through the top of the tank and, simultaneously, unloading by gravity through the bottom at the same flow rate. In June 1998, a full scale trial was performed on the oil carrier M/V Lavras (Petrobras) to assess the efficiency of this method on a segregated ballast tank (2,286m3). The ballast water was taken while the ship was anchored on the coast of Pará State, Brazil (local depth = 15m). The ballast exchange was carried out en route, at 200 n.m. offshore (local depths > 2000m). Characterization of coastal and oceanic waters (controls) was done by casting sampling devices from amidships. A simulation model based on the dilution rate of methylene blue established 8 sampling points in the tank, representing areas of average and low exchange rates. Sampling from the tank was done with a pneumatic pump (20mm-diameter hose, flow = 10L/min) which was efficient for phytoplankton (concentrated in a 20µm mesh), but zooplankton sampling required tows (200µm mesh) through a manhole. Sediment from the empty tank was sampled before and after the experiment. The amount of the original water that remained after exchanging 3 tank volumes (21 hours) depended on the parameter analyzed: chlorophyll a (14%), methylene blue (10%), phytoplankton abundance (4%); only oceanic zooplankton groups with the dominance of oceanic copepods were found, and microalgae cysts/resting spores were close to non detected in the water column. The sediment was not quantified, but visual observation after deballast indicated that the thick layers previously present had been washed. Cysts/resting spores that remained in the tank (1-2 x 105 cysts-spores/L) indicate that organisms found in the sediment represent a problem for further investigation.