For identifying the mosses collected in different localities of the Malaysian region, the need was felt for a key to the genera. In the preliminary one that I constructed to this end the genera were taken in the delimitation accepted in the second edition of Brotherus, Natürl. Pflanzf. In addition to the latter the genera published after 1925 and therefore not included in Brotherus 1. c. are taken into account. In revising the families for Flora Malesiana I will doubtless be compelled to alter the position of some of the species and the delimitation of some of the genera, and at the end of series III of Flora Malesiana, which will contain the Mosses, I therefore intend to give a final key. I sincerely hope that the preliminary key will in the meantime have been tested by different bryologists, and that they will let me profit by their remarks. For this reason it is published here. The analytical key is based as far as possible on vegetative characters, especially on the shape of the leaf cells. The principal features of the sporophyte are noted, but are not, as a rule, made use of as alternatives. This applies particularly to those alternatives that lead to the main groups. Only when no reliable vegetative characters could be found, have characters of the sporophyte, especially those of the peristome, been used. The habitat of each genus, not its distribution in the Malaysian region, is indicated in the key.