The colony size and caste composition of different-sized Macrotermes gilvus (Hagen) (Isoptera: Termitidae: Macrotermitinae) mounds on Penang Island, Malaysia, were studied. The total number of termite individuals differed significantly among different-sized mounds. Small mounds contained 15,400 ± 600 (n = 3) individuals; medium mounds contained 33,500 ± 2,400 (n = 3) individuals; and large mounds contained 61,400 ± 4,400 (n = 3) individuals. Larvae constituted the largest proportion (42.60%) of the total population, followed by workers (42.19%), soldiers (14.16%), presoldiers (0.71%), and alates (0.34%). Population parameters (i.e., total population, total adult apterous caste, and total juvenile apterous caste) were positively and significantly correlated with mound size (e.g., mound height and mound diameter) and queen weight. Significant correlations also were found between mound parameters and queen weight. The sex ratio of alates was significantly skewed toward females in five of seven colonies. Of 44 colonies surveyed, 13.6% were polygynous and 11.4% were polyandrous. The degree of physogastry of female reproductives in polygynous colonies was lower than that in monogynous colonies. In contrast, there was no significant difference in the mean weight of male reproductives between monoandrous and polyandrous colonies.