In plants, algae and some photosynthetic bacteria, the ElectroChromic Shift (ECS) of photosynthetic pigments, which senses the electric field across photosynthetic membranes, is widely used to quantify the activity of the photosynthetic chain. In cyanobacteria, ECS signals have never been used for physiological studies, although they can provide a unique tool to study the architecture and function of the respiratory and photosynthetic electron transfer chains, entangled in the thylakoid membranes. Here, we identifiedbona fideECS signals, likely corresponding to carotenoid band shifts,in the model cyanobacteria Synechococcus elongatusPCC7942 and Synechocystis sp.PCC6803. These band shifts, most likelyoriginating from pigment(s) located in photosystem I,have highly similar spectra in the two speciesand can be best measured as the difference between the absorption changes at 500-505 nm and the ones at 480-485 nm. These signals respond linearly to the electric field and display the basic kinetic features of ECS as characterized in other organisms. We demonstrate that theseprobes are an ideal tool to studyphotosynthetic physiologyin vivo, e.g.the fraction of PSI centres that are pre-bound by plastocyanin/cytochrome c6in darkness (about 60% in both cyanobacteria, in our experiments),the conductivity of the thylakoid membrane(largely reflecting the activity of the ATP synthase) orthe steady-state rates of the photosynthetic electron transport pathways.