Selective uptake of various dyes into an aggregate of amphiphilic copolymer consisting of a hydrophilic linear polyelectrolyte block and hydrophobic block carrying pendant dendritic moiety has been investigated in water. The copolymer associated into an acraregate with a hydrophobic interior at concentrations above 0.2 mg cm(-3). The uptake (23 and 36 molecules per aggregate, respectively) of pyrene and Oil Yellow in an aggregate was one order higher than that of benzo[a]pyrene and SudanIII. The hydrophobic dyes are always doped in the interior of the aggregate, but the difference in uptake among dyes may depend on their structure. Even if a large number of guest molecules was doped into the interior of an aggregate, the size of the sphere-like aggregate was conserved. It is suggested that guest molecules are encapsulated into the persisting cavity within and between hydrophobic dendron moieties in an aggregate. Structure-selective uptake reported in this investigation is a unique character of an aggregate of copolymer with dendron moiety because the dendron moiety offers a large void for doping.