We present a three-band k.P Hamiltonian to describe the conduction band dis
persion of ordered GaNxAs1-x crystals for low N concentrations (x less than
or similar to 0.05). The model includes interactions between the highest v
alence band, lowest conduction band and a higher-lying band formed by N res
onant states. The k.P conduction band dispersion is in excellent agreement
with full tight-binding calculations, and can be used as a basis for a wide
range of studies of Ga1-yInyNxAs1-x heterostructures and devices.