The natural remanent magnetization of a minor ultrabasic dike on the Island of Ytteröy, Norway, was investigated with a view to obtaining information about the time of intrusion. The dike dissects Ordovician limestone but is not affected by the Caledonian orogeny. A Permian age has previously been suggested. The rock is unique in some respects in that it exhibits selfreversal properties both in nature and in the laboratory during thermal demagnetization. Nevertheless, a fairly stable magnetic direction has been derived. The result does not confirm a Permian age but agrees with the direction of the Late Caledonian geomagnetic field as deduced from other Norwegian rocks. The problems of dating a smaller rock body by means of palaeomagnetism are discussed in some detail.