A flux monitoring set-up is described, based on wavelength-modulated atomic absorption spectroscopy (WMAAS) in-situ real-time measurements. It is particularly suited for element III fluxes monitoring during the growth of 111-V semiconductors and heterostructures inside a molecular beam epitaxy (MBE) chamber. The tunable source is a blue-violet laser diode mounted in an external cavity, followed by an interferometer required to actively stabilize the emitted wavelength. Measurements have been carried out for the Ga element at 403.3 nm, with a growth rate in the range 0.027-1.5 ML/s on GaAs. The WMAAS signal shows a linear dependence on the growth rate over the whole range studied here. The uncertainty and low flux limit are measured to be 0.01 ML/s at most. (c) 2006 Elsevier B. V. All rights reserved.