The behavior of a helical spin configuration of a hexagonalcrystal in an applied field is discussed for the case where the axisof the helix is identical with the hexagonalcaxis and the magnetizationvector in each layer is parallel to the basal plane. In a smallfield applied parallel to the basal plane, a slight deformation ofthe helix occurs resulting in a small increase in over-all magnetization.If the field surpasses a certain critical value, the helix changesabruptly into a state with high resulting magnetization. In thisstate the spin directions are oscillating about the direction of theapplied field, the magnetization being about 85% of the saturationvalue. In still higher fields, saturation is reached completely. Ahelical spin configuration has been found inMnAu2by Herpin,Me´riel, and Villain. We have investigated the properties of thehexagonal oxide(Ba,Sr)2Zn2Fe12O22which is of the Y-type. TheBa-rich composition is ferrimagnetic and has a preferential planefor the magnetization at all temperatures. The Sr-rich compositionhas no spontaneous magnetization, but a magnetization equal tothe one in the Ba case can easily be induced by an applied field.This behavior can be explained by the assumption of a helicalspin configuration. As an other example, we show that the magneticproperties of dysprosium as investigated by Behrendt,Legvold, and Spedding can be explained by assuming a helicalspin configuration between 85°K and 178°K. The angle betweentwo neighboring layers is equal to zero at 85°K, the ferromagneticCurie temperature, and increases at higher temperatures. Themagnetization process mentioned above is modified slightly indysprosium by the presence of a strong magnetic anisotropy in theplane. In the presence of this anisotropy the transition betweenthe ferromagnetic state and the helical state turns out to be afirst-order transition, which explains the observed specific heat.