We investigated the mass and size distributions of aggregates formed under low shear (14 s(-1)) by collisions between hydrated colloids of micrometric size under marginal stability conditions, The mass c(n) and size c(d) distributions determined by particle counting and laser diffractometry respectively were found to scale with n(-tau) and d(-alpha). Aggregation/fragmentation processes rapidly established a quasi dynamic equilibrium situation where, however, a prevailing aggregation and fragmentation process was successively determined to occur. Under marginal stability conditions, the particle concentration and ionic strength differently induced the aggregate formation. In 0.15 M NaCl suspension, below a threshold latex concentration, an increase in the particle concentration induced an increase of the aggregate average masses while above this value, a decay of the average masses was determined. In 0.075 M NaCl suspensions, where the influence of the van der Waals attraction was slightly reduced by the effect of the electrical repulsion, an increase of the particle concentration induced a decay of the aggregate average masses. Preliminary aging of the aggregates over a critical time was assumed to be required for establishment of cohesive interparticle links. Correlation between the reduced mass and size distributions unequivocally led to the fractal dimension f = alpha/tau of the aggregates. In 0.15 M NaCl suspensions the fractal dimension was determined to be 1.4 while in 0.075 M NaCl suspensions, aging and restructuration led to aggregates of fractal dimension 1.8. These relatively low values of the fractal dimension were interpreted by the existence of a preferential orientation of the aggregates in the low shear flow.