We present observations on the breeding patterns in a free-ranging population of common marmosets, Callithrix jacchus We found evidence of the simultaneous presence of two reproductively active females in all three monitored groups. We recorded at least one full reproductive cycle (conception-birth) for two females in each group, but other adult females did not breed. An overview of data from other callitrichid field studies indicates that the observed pattern may be an additional characteristic of the flexibility of the social organization of common marmosets. Comparisons with data from other sites where callithrix has been studied indicate that the presence of a second breeding female in a group may be related to high population density, though the underlying mechanisms remain unclear. © 1994 Plenum Publishing Corporation.