Fluorescently labeled amphiphilic polybetaines have been prepared, and their dilute solution properties in water, chloroform, methanol, and mixed organic solvents have been investigated by fluorescence spectroscopy, dynamic light scattering, and H-1 NMR spectroscopy. The copolymer chains consisted of N-isopropylacrylamide and N-phosphorylcholine-N'-ethylenedioxybis(ethyl)acrylamide units in similar to1/1 molar ratio, as well as from 3 to 10 mol % hydrophobic units, such as N-n-(octadecyl)acrylamide, N-(1H,1H-perfluoro-n-octyl)acrylamide, or N-[(1-pyrenyl)-4-butyl]-N-(n-octadecyl)acrylamide. In water; the polybetaines associate in multichain aggregates ranging in size from 170 to 220 nm. The assembly of the polymers in water is triggered by two cumulative effects: (1) hydrophobic interactions between the hydrocarbon or fluorocarbon chains and (2) ion pair formation between the phosphorylcholine groups. The polymers do not associate in methanol, but chain aggregation occurs in chloroform, where the phosphorylcholine groups form-clusters stabilized by water structuration around these highly polar groups.