Absorption and fluorescence spectra are reported for chlorophyll a incorporated into a number of aqueous phospholipid dispersions. Absorption spectra show that in dipalmitoylphosphatidylcholine bilayers, monomeric and oligomeric forms of chlorophyll a are present in both the gel and liquid crystalline phases. The formation of aggregates of chlorophyll a is reflected in the fluorescence spectra by a marked concentration quenching. In bilayers conatining small proportions of chlorophyll a, a marked increase in aggregation occurs at the transition temperatures that can be detected calorimetrically. At higher concentrations (greater than 1 chlorophyll:100 lipid), the "pretransition" is abolished in the phosphatidylcholines, and the main transition is broadened, consistent with an orientation for the chlorophyll a with the chlorine ring in the head group region and the phytol chain in the fatty acid chain region of the bilayer. In mixtures of saturated and unsaturated lipids, there is no preferential segregation of the chlorophyll a into the unsaturated lipid.