Ngazidja Island in the Comoros archipelago (in the south-western region of the Indian Ocean) is an example of a matrilineal and matrilocal Muslim society. This article addresses the various ways in which the Ngazidja people negotiate gender and family relations by referring to matrilineal or Islamic norms and values, and the influence of Islamic reforms and Western norms. Matriliny is linked to a local political organization based on an age system and a male political assembly. This link explains the specific way in which matrilineal and Islamic ideologies are linked. Marriage is ruled by Islamic law, but alliances between matrilineal descent groups follow other rationales. The existence of two matrimonial regimes illustrates this distinction. The conceptions of fatherhood produced in Ngazidja shed light on the key issue of male authority in matrilineal and Muslim contexts.