The kinetics and mechanisms of aggregate formation and disruption have been investigated for BHK natural aggregates grown in 250 cm(3) Coming spinner flasks. The cells were cultivated as continuous cultures with a dilution rate of 0.005 day(-1). Hydrodynamics were manipulated varying the agitation rate in a single step, and aggregate characteristics were evaluated until a new stationary state was achieved. Steps increasing agitation rate from 30 to 90 and 100 rpm (for disruption studies) and decreasing from 100 to 30 45 and 60 rpm (for formation studies) were performed. The initial and final size distributions are essentially monomodal, with bimodal size distributions being observed during the transition phase. Comparing the 30 and 100 and 100 to 30 rpm tests, almost complete reversibility in aggregate size was observed. Aggregates are hydrodynamically controlled, with a power dependence of aggregate size on the energy dissipated per unit of mass close to 0.25. Having dimensions smaller than the Kolmogoroffs eddy size, aggregates are controlled by the action of viscous stresses in the viscous dissipation subrange. The variation in aggregate diameter and concentration with time has been mathematically described, based on kinetic models (first-order for diameter, first- and second-order for concentration), derived from several theories and correlations previously applied to the formation and disruption in different aggregate systems. Aggregate formation is a faster process than disruption; disruption is occurring with two main mechanisms : breakage and cell shedding from the large aggregates, leading to smaller aggregates and an increase in the fraction of single cells. The breakage of large aggregates into smaller ones can occur due to the collisions between aggregates or due to the stress caused by fluid motion. The increase on the agitation step leads to an increase on hydrodynamic stress, and thus the collisional fragmentation mechanism becomes less relevant. Formation is occurring by aggregate-aggregate collisions and, with a smaller relevance, single cell addition into the aggregates. Nevertheless reduced in number, more efficient collisions are occurring between aggregates, leading to an increase in the rate of aggregate formation.