The self-assembly properties of an extracellular material of glycoprotein character produced by a new Gram-negative species, NF3, Pseudoalteromonas antarctica, isolated from muddy soil samples of Antarctica have been investigated. The aggregation behavior of this exopolymer was studied directly by transmission electron microscopy (TEM) and analysis of digitalized TEM images of its aqueous dispersions before and after sonication. Increasing amounts of glycoprotein (GP) in water led to an abrupt decrease in the dispersion surface tensions up to a GP concentration of about 0.20 mg/ml (from 72 to 47 mN m(-1)), followed by an almost constant surface tension value. The size distribution curves of the aggregates formed always showed a bimodal distribution. The mean size of these two aggregates increased as GP concentration increased (first peak from 120 to 140 nm and second peak from 500 to 700 nm), reaching in both cases almost a constant value also for 0.20 mg of GP/ml of water. TEM images of unsonicated aqueous GP dispersions at concentrations lower and higher than 0.20 mg/ml always showed the coexistence of concentric multilamellar and small unilamellar aggregates, the small particles being the dominant class in the first case. Sonication of these dispersions revealed that each lamella of the initial multilamellar structures was made up of various subunits of coiled coil, whereas the smaller particles were not composed of these subunits. Profiles from digitalized TEM images of unsonicated and sonicated dispersions confirm that each lamella of large aggregates was composed of three subunits.