The mutual forces between two spheres having their line of centers parallel to the direction of propagation of traveling plane waves may be many times greater than the usual radiation forces of repulsion exerted on a single sphere. Expressions are derived for the forces on each sphere in terms of diverging wavefunctions and coordinate‐transformation functions. Numerical calculations show that the mean force on the sphere nearest the sound source has a value which oscillates with increasing amplitude as the separation between spheres is decreased, reaching a maximum value of repulsion from the source when the spheres are a quarter‐wavelength apart. At smaller separations mutual repulsion between the spheres dominates. For the other sphere, the forces are always of repulsion, but increase in magnitude as the spheres are brought together.