A template-directed mechanism of crystal growth is demonstrated for urea inclusion compounds (UICs). For UICs containing n-alkanone or alpha+1,omega-1-alkanedione guests, x-ray diffraction revealed superstructure relations between host and guest repeats along the channel axis for guests containing 8 to 14 carbons, For a favorable structural match between host and guest, UICs typically grow as flat hexagonal plates, and atomic force microscopy of {001} surfaces revealed molecularly smooth terraces differing in height by multiples of the guest repeat, If the match is poor, protrusion of guests from the {001} surfaces nucleates growth along the channel axis to form hexagonal needles.