In the course of my study of the Thymelaeaceae for the Flora Malesiana, it was surprising to find that the well known Asiatic species Aquilaria agallocha Roxb. is very similar to the Malesian A. malaccensis Lamk, and that the Chinese species Ophiospermum sinense Lour, was transferred to Aquilaria independently by Sprengel (1825), Gilg (1894), and Merrill (1920), with the specific epithet either ‘chinensis’ or ‘sinensis’. In order to clarify the status and delimitation of the species concerned, the results of my investigations may follow here. Among the unnamed collections of Thailand Thymelaeaceae received for determination from the Kew Herbarium, two species of Aquilaria were found, a new one, A. subintegra Ding Hou, and a new record for the flora of that country, A. crassna Pierre ex H. Lec.