Abstract Complex sociality is widespread in lizards, but the difficulties of directly observing social interactions in free-ranging snakes have precluded such studies for most snake species. However, a type of data already available from mark-recapture studies (dates of capture and recapture of individually marked animals) can reveal social substructure within snake populations. If individuals associate with each other in social groups, we expect synchrony in the dates of capture and recapture of those animals. A field study of turtle-headed sea snakes (Emydocephalus annulatus) in New Caledonia reveals exactly this phenomenon. For example, animals that were captured on the same day in one year often were recaptured on the same day the following year. Analysis rejects non-social interpretations of these data (such as spatial-temporal confounding in sampling, intrapopulation heterogeneity in cues for activity), suggesting instead that many individual sea snakes belong to ‘social’ groups that consistently move about together. The phenomenon of capture synchrony during mark-recapture studies can provide new insights into the occurrence and correlates of cryptic social aggregations.