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WO circumstances made the present study of the chromosome mor-T phology of Pinus silvestris necessary. 1) Although many cytological investigations have concerned this and other Pinus spp. (SAX and SAX, 1933; MERCEN and NOVOTNY, 1957;AASS, 1957;HAKANSSON, 1956), there is no critical study of the karyotype of Pinus silvestris. 2) Since this species is increasingly used for radio-biological studies involving cytological observation (SUSZKA a.o., 1960), a clear understanding of its basic karyotype is essential. Whereas the lack of a satisfactory technique seems to have been the main difficulty in earlier studies, several combinations of pretreatments and fixations, applied in the present study, appeared to constitute a useful technique. As we have been able to obtain satisfactory results by our method, we hope that this description will be of value for investigators working with other gymnosperms as well. MATERIAL AND METHODSPrimary root nieristems of recently germinated seeds were used. The material was collected in Sweden from Bogesund (59'24') and Kiruna (67O51'). Since even the metaphase chromosomes of Pinus are generally very long, they are easily broken with ordinary chemical pretreatments or light physical pressure during the process of squashing. Extreme care must therefore be exercised in the choice of pretreatment and pressure to be applied during the preparation of the slide. The following technique, which is a modification of the one previously used (HHADURI