The introduction of Islamic criminal law in twelve northern states of the Nigerian federation after 1999 was widely perceived as an attempt to Islamise the Nigerian state. In this article it is argued that the “šarī'a project” started as a pre-election promise, but was immediately supported by Muslim reform groups whose aim was not the establishment of an Islamic state but rather the imposition of šarī'a compliant behaviour on Muslims. Particular emphasis was put on illicit sexual relations (zinā). However, Muslim societies of northern Nigeria have a notion of zinā which differs in important aspects from the classical doctrine, and certain forms of socially accepted extramarital sexuality still exist. Based on an analysis of a sample of šarī'a court trials for rape, sodomy, incest and zinā, it is shown that the judicial practice in šarī'a courts has helped to mitigate the effects of Islamic criminal law on the traditional societies in northern Nigeria. In particular, accusations based on suspicion and pregnancy out of wedlock as proof of zinā have been rejected by the courts, thereby confirming the privacy of the family compound and traditional conflict resolution through mediation. At the same time, male control over female sexuality has been strengthened.