The dynamics and formation mechanisms of marine snow aggregates from abandoned larvacean houses were examined by laboratory experiments and field sampling during a spring diatom bloom in a shallow fjord on the west coast of the USA. Intact aggregates were sampled both from sediment traps and directly from the water column by divers. ALI aggregates were composed of 1 abandoned house of the larvacean Oikopleura dioica to which numerous diatoms, fecal pellets, ciliales, and amorphous detritus were attached. High vertical flux rates (20000 to 120000 houses m(-2) d(-1)) and settling velocities (average 120 m d(-1)) imply a rapid turnover of suspended larvacean houses, and concentrations of diatoms and fecal pellets in the aggregates exceeding ambient concentrations by 3 to 5 orders of magnitude suggest their potential importance in driving the vertical flux of particles. Identical particle assemblages were observed in aggregates collected in the water column and in sediment traps. Most of the fecal pellets found in the houses were most Likely produced by the larvaceans themselves. Numbers of diatoms per house corresponded with the diatom concentrations in the ambient water and, on average, each aggregate contained diatoms in abundances corresponding to those found in 4.5 ml of ambient water. Laboratory measurements showed that larvacean houses scavenge diatoms and fecal pellets while sinking, and observed scavenging rates were similar to those predicted from theory. However, both predicted and observed scavenging rates in experiments were orders of magnitude too low to account for the particle content observed on aggregates from the water column. Based on models, shear coagulation was also assessed to be insignificant in forming aggregates. It is concluded that most of the particles become attached to the incurrent filters of the larvacean house while it is still inhabited, and that shear and sinking insignificantly contribute to particle collisions and adhesions on the abandoned house.