Ancient populations have commonly been thought to have lived in small groups where extreme endogamy was the norm. To contribute to this debate, a genetic analysis has been carried out on a collective burial with eight primary inhumations from Montanissell Cave in the Catalan pre-Pyrenees. Radiocarbon dating clearly placed the burial in the Bronze Age, around 3200 BP. The composition of the group—two adults (one male, one female), one young woman, and five children from both sexes—seemed to represent the structure of a typical nuclear family. The genetic evidence proves this assumption to be wrong. In fact, at least five out of the eight mitochondrial haplotypes were different, denying the possibility of a common maternal ancestor for all of them. Nevertheless, 50% of the inhumations shared haplogroup J, so the possibility of a maternal relationship cannot be ruled out. Actually, combining different analyses performed using ancient and living populations, the probability of having four related J individuals in Montanissell Cave would range from 0.9884 to 0.9999. Owing to the particularities of this singular collective burial (small number of bodies placed altogether in a hidden cave, the evidence of non-simultaneous interments, close dating and unusual grave goods), we suggest that it might represent a small group with a patrilocal mating system. Am J Phys Anthropol, 2011. © 2011 Wiley-Liss, Inc.