A sufficient condition for instability of the Euler equations [see (1.5)] is used to demonstrate the instability of certain ABC flows. In the parameter rangeA= 1,B2+C2> 1, instability follows from the presence of hyperbolic stagnation points. In the parameter rangeA= 1,B=C= «ϵ1, instability follows from the existence of hyperbolic closed trajectories and the associated exponential stretching of the fluid particles. This result is proved analytically in Section 2 and illustrated numerically via Poincaré sections in Section 3.