The effect of aggregation on the photophysical properties of three cationic poly{9,9-bis[N,N-(trimethylammonium)hexyl] fluorene-co-1,4-phenylene} polymers with average chain lengths of similar to 6, 12, and 100 repeat units (PFP-NR3(6(1),12(Br),100(Br))) has been studied by steady-state and time-resolved fluorescence techniques. Conjugated polyelectrolytes are known to aggregate in solution and for these PFP-NR3 polymers this causes a decrease in the fluorescence quantum yield. The use of acetonitrile as a cosolvent leads to the breakup of aggregates of PFP-NR3 in water; for PFP-NR3(6(1)), this results in an similar to 10-fold increase in fluorescence quantum yield, a ca. 2-fold increase in the molar extinction coefficient at 380 nm, and an increase in the emission lifetime, as compared with polymer behavior in water. Fluorescence anisotropy also decreases with increasing aggregation, and this is attributed to increased fluorescence depolarization by interchain energy transfer in aggregate PFP-NR3 clusters. Forster resonance energy transfer along the polymer chain is expected to be very fast, with a calculated FRET rate constant of 7.3 x 10(12) s(-1) and a Forster distance of 2.83 nm (cf. the polymer repeat unit separation of 0.840 nm) for PFP-NR3(100(Br)). The complex polymer excited-state decay kinetics in aggregated PFP-NR3 systems have been successfully modeled in terms of intrachain energy transfer via migration and trapping at interchain aggregate trap sites, with model parameters in good agreement with data from picosecond time-resolved studies and the calculated theoretical Forster energy-transfer rates.