Lamellar single crystals of the aromatic polyketone PK99 have been grown from dilute solution in benzophenone. The crystals are multilayer and shaped like extended leaves, they then twist about their long axes. Selected area electron diffraction and wide angle X-ray diffraction (WAXD) of crystal mats show that the unit cell is orthorhombic with a = 0.788 nm, b = 0.609 nm and c = 4.82 nm. This structure is also found for the melt drawn fibre and is similar to that suggested by other authors who looked at melt crystallized material. The unit cell is slightly larger than the unit cells of PEEK and PEK in the h k 0 plane. Small angle X-ray diffraction of solution grown crystals indicates a long period of 7.86 nm, this thickness is close to one and a half times the chain repeat. The growth rate from the melt has been investigated as a function of crystallization temperature, using both optical and transmission electron microscopy (TEM). Results from the two techniques agree within the experimental errors. As has been found by other authors, there is a maximum growth rate at 260 degrees C, a minimum between 235 and 240 degrees C and a second, much lower, maximum at 230 degrees C.