The formation of periodic waves in the near wake of straight circular cylinders with arbitrary aspect ratios in crosswise flows at low Reynolds numbers (Re) is analyzed. Using a semiempirical model it is shown that raising the Reynolds number beyond a critical value of about Re(1)=22 yields a supercritical Hopf bifurcation preceding the known onset of the von Ka´rma´n vortex flow at about Re(2)=46.7. In agreement with experimental trends a stable periodic, secondary transversal motion arises in the previously steady crosswise flow.