Neutron diffraction studies on polycrystalline and singlecrystal specimens of thulium were made at room temperature to 1.3 deg K. The results are interpreted by a method exhibiting explicitly the Fourier components of the distribution of magnetic moments on the lattice sites. At about 56 deg K, the Neel temperature, a simple oscillating zcomponent-type antiferromagnetic structure is developed. At approximately 40 deg K, nonzero Fourier coefficients of overtones of the fundamental observed at higher teraperatures are first detected. At 4.2 deg K, the magnetic structure of thulium is a type of antiphase domain structure in which several layers of moments parallel to the + a/sub 3/ direction are followed by several layers in which the moments are oppositely directed. The sequence in thulium is --4, +3, --4, +3,... etc. Each atom has, within the precision of the experiments, an ordered moment of 7 mu s, and the ferrimagnetic structure has a net moment, parallel to the c axis, of 1 mu /sub B/ per atom. The fundamental period of the modulation remains constant over the whole range of teraperatures at a value corresponding to 3.5 a/sub 3/ periods. The scattering amplitude of thulium is b/sub Tm/ = (0.69 plus or minusmore » 0.02) x 10/sup -12/ cm. (auth)« less