Snakes are often regarded as the least social of all vertebrate groups, but this assumption stems from the fact that they are secretive and difficult to observe in nature, rather than direct evidence. Recent studies have revealed a surprising degree of social complexity in snakes. Here, I examine the ability of captive-raised timber rattlesnakes (Crotalus horridus) to recognize siblings by measuring the mean separation distance and frequency of contact between pairs of individuals housed together. The results show that female siblings associate more closely with each other than nonsibling pairs. Previous studies have shown that timber rattlesnakes occupying the same hibernacula have higher relatedness than snakes using neighbouring hibernacula, and frequently form social aggregations. Rattlesnakes exhibit other characteristics consistent with advanced sociality, including group defence, conspecific alarm signals and maternal defence of young. These findings reinforce the notion that, rather than being solitary and asocial, some snake species may form family groups.