The magnetic retentivity of many materials is about half of the magnetization at saturation, a fact accounted for by simple domain theory. In some materials, however, the retentivity is only a small fraction of saturation, sometimes less than 10 per cent. The explanation of this fact is discussed. It is suggested that in materials with almost zero magnetic anisotropy the Bloch walls between domains increase in thickness until they envelope the whole specimen and the domain structure disappears.