Multiplicity of phase states within frequency locked bands in periodic
ally forced oscillatory systems may give rise to front structures sepa
rating states with different phases. A new front instability is found
within bands where omega(forcing)/omega(system) = 2n (n > 1). Stationa
ry fronts shifting the oscillation phase by pi lose stability below a
critical forcing strength and decompose into n traveling fronts each s
hifting the phase by pi/n. The instability designates a transition fro
m stationary two-phase patterns to traveling n-phase patterns.