Plant protoplasts treated in the same way as lymphocytes in animal cytogenetics allowed to point out a chromosomes counting technique in tobacco tissue cultures (MOURAS 1975). The analysis of the factors important to develop this method (pretreatment, osmotic concentration and temperature of the cell-enzyme mixture) leads to an improvement of the method. Hence it becomes possible to consider its application in order to identify not only mitotic chromosomes of tobacco, for which a karyogram is presented (Fig. 1), but also those of species with which the squash technique has only given, till now, poor results:Prunus(Figs. 2 and 3) andCorylus.