The present work explores the potential use of the conjugated cationic polyfluorene {[9,9-bis(6'-NAN-trimethylammonium)-hexyl]fluorene-phenylenel bromide (HTMA-PFP) as a fluorescent membrane marker. To this end, the interaction of the polyelectrolyte with anionic model membranes has been investigated using different biophysical approaches. High affinity interaction was confirmed through alterations in the fluorescence spectrum of HTMA-PFP and by Forster resonance energy transfer (FRET) analysis. Quenching data indicate that once HTMA-PFP interacts with the membrane, it penetrates in the hydrophobic core embedded in the lipid bilayer where it presents high fluorescence quantum yield and photostability. Leakage experiments and dynamic light scattering (DLS) measurements show that the integrity of the lipid vesicles is maintained after polymer incorporation since no vesicle fusion or decomposition into small fragments is detectable. This conclusion is supported by fluorescence microscopy images, which confirm that polyelectrolyte interacts with the vesicle, labeling the lipid membrane without altering its morphology. Further experiments performed as a function of temperature indicate that the polymer is accommodated in the membrane without inducing significant loss of lipid cooperativity and without altering the packing of lipids within the bilayer. Finally, results show that polyelectrolyte fluorescence is sensitive to the large structural changes taking place in the lipid bilayer at the lipid phase transition. All these results confirm the ability of HTMA-PFP to visualize membrane structures and to monitor membrane processes.