The fluorescence depolarization associated with the hydrophobic fluorescent probe 1,6-diphenyl-1,3,5-hexatriene is used to monitor the changes in fluidity accompanying the gel-liquid crystalline phase transition in synthetic phosphatidylcholine dispersions. The parameters of the phase transition are determined for both large, multilamellar liposomes and small, single-lamellar vesicles. These parameters are compared with those obtained using other techniques. The data are interpreted in terms of 2 limiting molecular models, which in turn offer insight into the structural differences between multilamellar liposomes and small vesicles.