Mn-doped GaAs is a ferromagnetic semiconductor(1,2), widely studied because of its possible application for spin-sensitive 'spintronics' devices(3,4). The material also attracts great interest in fundamental research regarding its evolution from a paramagnetic insulator to a ferromagnetic metal(5,6). The high sensitivity of its physical properties to preparation conditions and heat treatments(7,8) and the strong doping and temperature dependencies of the magnetic anisotropy(9,10) have generated a view in the research community that ferromagnetism in (Ga, Mn) Asmay be associated with unavoidable and intrinsic strong spatial inhomogeneity. Muon spin relaxation (mu SR) probes magnetism, yielding unique information about the volume fraction of regions having static magnetic order, as well as the size and distribution of the ordered moments(11-13). By combining low-energy mu SR, conductivity and a. c. and d. c. magnetization results obtained on high-quality thin-film specimens, we demonstrate here that (Ga, Mn) As shows a sharp onset of ferromagnetic order, developing homogeneously in the full volume fraction, in both insulating and metallic films. Smooth evolution of the ordered moment size across the insulator metal phase boundary indicates strong ferromagnetic coupling between Mn moments that exists before the emergence of fully itinerant hole carriers.