Perikinetic and orthokinetic aggregation in laminar tube flow of hydrophilic polystyrene latex particles was investigated to determine the aggregation characteristics of weakly unstable colloids in aqueous media. The weight S and number N average sizes of colloids and the colloid mass frequency c(n) were determined by particle counting, Destabilization of hydrated latex particles was found to be predicted by DLVO theories as far as the repulsive forces of the hydration layer were assumed to oppose the van der Waals attractive forces, the combined effects being expressed using an apparent low Hamaker constant, Deviation from the usual reaction-limited aggregation process could be established by comparing experimental and theoretical power laws for perikinetic processes. The development of orthokinetic processes depended on latex concentration and shear rate, At a given mean shear rate of 106 s(-1) the aggregation rate increased with the latex concentration below 0.3 g/L, while above this concentration the mass of aggregates collected at the tube exit strongly decreased. Hydrodynamic interactions similarly modified the collision efficiency of the smallest colloids, whereas the volume flow rate slightly modified the mass distribution of aggregates of intermediate and large sizes. The true collision efficiency in laminar tube flow could be established by taking into account the effective volume fraction of aggregates.