We investigated whether attached bacteria affect the settling velocity of organic aggregates via modifications of the physical properties of aggregates, including density, porosity, and morphology. Model aggregates, prepared by mixing 2 different polysaccharides (fucoidan and chitosan), were incubated in coastal seawater passed through either 0.8 μm (AGG 0.8 ) or 0.2 μm (AGG 0.2 ) filters. After incubation for 48 h, AGG 0.8 were much more densely (3.2-to 10.1-fold) colonized by bacteria than AGG 0.2 . Based on median settling velocities (W 50 ), as determined by laser in situ scattering and transmissometry, the W 50 of AGG 0.8 was lower (1.6-to 4.5-fold) than that of AGG 0.2 for a size class of 62 to 119 μm. Stokes model analyses indicated that this reduction in W 50 could be largely attributed to the higher porosities of AGG 0.8 (0.932−0.981) than those of AGG 0.2 (0.719−0.929). Our results support the notion that the modification of aggregate structure by attached bacteria (porosity enhancement) can be an important factor controlling the settling velocity of marine particles.