The somatic chromosomes, obtained from short term leukocyte cultures, were studied of four species of the Hylobatidae:Hylobates lar, H. agilis, H. (Nomascus) concolor and Symphalangus syndactylus. In accordance with earlier observations by others, the diploid chromosome numbers were found to be 44 in both Hvlobates lar and H. aeilis, 52 in H. concolor and 50 in Symphalangus syn.da&lus. The chromosome Passociations observed in metaphase spreads are clearly different in the three types of chromosome complements.In Hylobates lar and H. agilis associations are found between both members of the marked chromosome pair. In Symphalangus syndactylus the only two acrocentric elements of the karyotype, which are of medium size, associate frequently.In H. concolor finally, the members of three pairs of small acrocentrics are involved in chromosome associations.G-banding patterns (obtained by trypsin treatment) showed that in a male individual of this species also the small acrocentric Y chromosome sometimes participates in these associations.The evolutionary aspects of these observations are briefly discussed. 9 L. E. M. DE BOER AND C. VAN OOSTRUM-VAN DER HORST