Interspecific comparisons between canids suggest allometries in the characteristics of neonates, such that species with lower body weights tend to produce relatively heavy neonates, in relatively small litters whose combined weight is relatively low. Canids weighing less than 6 kg have generally been reported as monogamous with occasional polygyny; on the basis of a parental investment hypothesis, the interspecific allometries suggest that the smallest canids have the greatest tendency to be polygynous. This prediction was explored by studying Blanford's foxes, which at ca 1 kg are lighter than any previously studied. Data from 11 radio-tracked foxes studied over 2 years indicated that they were organized as strictly monogamous pairs in territories of about 1·6 km2 which overlapped minimally. Three out of five territories contained a single non-breeding yearling female during the mating season, but there was no evidence of polygyny. It is suggested that the departure of Blanford's fox from the allometric trends can best be explained in terms of adaptations to its habitat rather than as a direct consequence of small size.