The present study investigates vertical distribution of fish size in three 12 m  ×  12 mwide and 14 m deep sea cages stocked with Atlantic salmon of average weight from 3.5 to3.7 kg, at commercial densities between 15.6 to 16.2 kg m-3, in Norway. Foreach cage, individual fish weight were estimated by three 0.6 m  ×  0.6 m measuring framesas fish swam through. The frames were positioned at 3, 6 and 9 m depth. Recordings werecarried out over five days per cage in succession, and during relatively stableenvironmental conditions in late autumn 2010. In all cages, measured fish were 15−25%smaller at 3 m compared to the average weights at 6 and 9 m depth. Largest average weightdifference between depths within one cage was 0.995 kg. The average weight at 6 and 9 mwas higher at night-time compared to daytime. Fish at the lower end of the weight spectrumwere predominantly registered at 3 m, while fish at the higher end of the spectrum weremostly registered at the greater depths. Fish of average size were well represented at allthree depths. The fact that smaller fish swam shallower may be ascribed to naturalbehavioural traits and introduce an important consideration in representative samplingwithin commercial sea cages. In biomass estimations and sea lice counts size-dependentvertical stratification needs to be accounted for.