The first study of the kinetics of Al atoms with O-2 at low and ultralow temperatures is reported. Rate constants have been measured from 295 to 23 K using the pulsed laser photolysis-laser-induced fluorescence (PLP-LIF) technique in a CRESU (Cinetique de Reaction en Ecoulement Supersonique Uniforme) apparatus. Rate constants were found to increase monotonically with decreasing temperature: k(T) = (1.75 +/- 0.10) x 10(-10)(T/298)(-0.25+/-0.02) exp[(7.3 +/- 3.0)/T] cm(3) s(-1). The rate constant for the reaction Al with O-2 has also been calculated as a function of temperature using a theoretical approach based on the adiabatic capture centrifugal sudden approximation (ACCSA) method, These calculations were found to be in good agreement with experimental results and were able to explain the observed temperature dependence by the open-shell nature of the reactants, Possible astrophysical implications are also discussed concerning the chemistry of aluminum atoms in the interstellar medium.