The structures of many photosynthetic pigment-protein complexes have now been determined(1-6), but a real understanding of the photosynthetic membrane at the molecular level will also require knowledge of the organization and dynamics of these complexes in the intact membrane. Using fluorescence recovery after photobleaching (FRAP)(7) and a scanning confocal microscope, we have made direct measurements in vivo of the lateral diffusion of light-harvesting complexes and reaction centres in the thylakoid membranes of the cyanobacterium Dactylococcopsis salina(8). We find that the phycobilisomes (the accessory light-harvesting complexes of cyanobacteria) diffuse quite rapidly, but that photosystem II is immobile on the timescale of the measurement, indicating that the linkage between phycobilisomes and photosystem LT is unstable. We propose that the lateral diffusion of phycobilisomes is involved in regulation of photosynthetic light-harvesting (state 1-state 2 transitions). The mobility of the phycobilisomes may also be essential to allow the synthesis and repair of thylakoid membrane components.