The athermal α → ω transformation in Zr-2 at.% Nb alloy has been investigated by transmission electron microscopy. Analysis of the selected-area diffraction pattern has shown that the orientation relationships between the omega and the parent α-phase in quenched Zr-2 at.% Nb alloy are the same as have been previously observed for the α → ω reaction in pure zirconium. Thus it was deduced that the direct α → ω transition has taken place in the alloy during cooling. The α-originated ω-particles were visualized using the dark-field technique. The formation of the athermal omega in the α-region of β-stabilized Zr-Nb alloy is discussed in terms of the relative positions of the free energy equilibrium curves T0β→α, T0β→ω, T0α→ω and the corresponding Msβ→α, Msβ→ω and Tsα→ω start curves. It is concluded that the omega phase can occur over a much wider range of alloy compositions than is usually recognized on the basis of β → ω transformation data.