Considerable variation in worker size exists among colonies of the Formosan subterranean termite, Coptotermes formosanus Shiraki. The breeding system of this termite species ranges from simple families, headed by a pair of reproductives, to extended families headed by varying numbers of neotenic reproductives. Thus, termites from different colonies have different degrees of inbreeding. Effects of inbreeding on the biology subterranean termite species have generally been considered negligible, but empirical experiments were largely lacking. In this study, we investigated the relationship of physical parameters of termite workers, such as body weight and head width with breeding structure, degrees of inbreeding and relatedness within colonies. Termite workers were collected from 12 to 19 colonies from three populations (Guangzhou, China; Oahu, Hawaii; New Orleans, Louisiana). Weight and head size of workers were determined. Workers were genotyped at five to eight microsatellite loci to determine the breeding system of each colony and the degrees of inbreeding and relatedness. In colonies of all three populations, head size was positively correlated with body weight of the workers and both parameters were correlated with the degree of inbreeding within the colony. Possible costs of inbreeding and inbreeding depression are discussed.