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For the first time, a successful growth of Ga1−xMnxAs layers on (100)GaAs substrates by metal-organic vapour-phase epitaxy is reported. Optical and magneto-optical spectroscopy of the E0, E1and E1+Δ1transitions is possible due to the high structural quality of the samples. A strong exchange interaction is found in the Ga1−xMnxAs layers between the magnetic moments of manganese and the excitonic states, which exhibits an effective ferromagnetic coupling. The Mn concentrations in the layers can be estimated by two different and independent optical methods (i) from the Zeeman splitting of the E0excitons and (ii) from the red-shift of the E1transition due to the p-doping with Mn.