Single-phase quasicrystalline alloys of the series Al_70-xMn₁₅Pd₁₅B_x (0<or=x<or=10) prepared by the melt spinning method have been shown to be of the icosahedral structure. Ferromagnetic resonance (FMR) studies have been undertaken in order to characterize the magnetic behaviour of these materials. The alloy with x=0 exhibits a single resonance peak and does not exhibit spontaneous magnetic order. Alloys with x>or=1 show the existence of two resonances due to two distinct Mn environments in the icosahedral structure. One resonance, resulting from non-magnetic Mn ions, shows FMR parameters which are temperature independent. The resonance resulting from magnetically ordered Mn ions shows an amplitude and field which is a function of temperature, and disappears above a critical temperature. FMR measurements are supplemented with X-ray diffraction, calorimetry and magnetization data. The magnetic behaviour of this alloy system is explained on the basis of Anderson`s theory of moment formation and the indirect coupling of 3d moments.