We have studied the kinetics of folding and membrane insertion of the outer membrane protein OmpA of Escherichia coli. In the native structure, its membrane-inserted domain forms a beta-barrel. The protein was unfolded in solubilized form in water/urea, and refolding was induced by dilution of urea and simultaneous addition of lipid vesicles. Three transitions along the folding pathway could be distinguished. Their characteristic times lie below a second, in the range of minutes, and in the range of an hour. The fast process corresponds to the transition from the unfolded state in water/urea to a misfolded state in water, the moderately slow process to a transition from the misfolded state to a partially folded state in the membrane, and the slow process to the transition from the partially folded to the native state. The partially folded state in the membrane is interpreted as the analogue of the molten globule state of soluble proteins.