The P2Os-content of 300 samples of lamprophyre, lamproid and some acid dyke rocks from three areas of the Bohemian Massif has been determined colorimetrieally. The values established range from 0.02% in microgranite to 2.38% in an alkaline minette. A negative correlation exists between PeO5 and SiO~, and A1203, respectively, a positive one between Pu05 and Ti02, CaO and F, respectively. In the alkaline dykes most of the calcium is found in apatite. The main factor controlling the distribution of P205 in the rocks studied is their saturation with aluminium. Theoretically, this may be explained according to Shcherbina's (1966) acidity concept. If the chemical composition of a dyke remains approximately constant, the same holds good also for phosphorus, in spite of considerable differences in the mode. In the course of the differentiation of the kersantite type magma, P20~ increases at first in agreement with the AleOa saturation of the magma, but falls off in the final stages. Due to local factors, there exist considerable differences between the P205-contents of the individual rock types of various areas.
1 Chemically, these rocks are similar to the dioritic rocks, but their plagioclase, insofar as it is not albitized, is a labradorite and bytownite.