Aggregation of poly(allylammonium) chloride (PAAH+Cl-) and sodium 1-octanesulfonate (Na+OcS-) has been examined by the energy transfer from 2-naphthalenesulfonate ion (2-NpS-) to 1-pyrenesulfonate ion (1-PyS-) using steady-state fluorescence spectroscopy. In aqueous PAAH+Cl- solution without Na+OcS-, both 2-NpS- and 1-PyS- are in an isolated state around the polyion under the conditions studied, indicating no energy transfer between them. In the presence of Na+OcS-, on the other hand, efficient energy transfer takes place between 2-NpS- and I-PyS-. In such a system, the aggregate of PAAH+ and OcS- is formed owing to the electrostatic interaction between PAAH+ and OcS- and the hydrophobic interaction among OcS- ions bound to the polyion. Both 2-NpS- and 1-PyS- are incorporated in close proximity in the hydrophobic domain of the aggregate, which leads to energy transfer between them. The concentration at which the aggregation of PAAH+ and OcS- occurs was about three orders of magnitude lower than the critical micelle concentration of OcS- alone. The fluorescence probe technique using pyrene has also been employed to monitor the aggregation.