This paper deals with a secret language (kalamo) spoken by the Anakara clan of the Antaimoro tribe in the south-east of Madagascar. According to their own tradition, they migrated to the island between the twelfth and the fifteenth centuries, from the Arabian peninsula. Their sacred writings (sorabe) are written in Arabic script in a mixture of Malagasy and Arabic. The secret language kalamo contains a large number of Arabic loanwords, as well as Malagasy words that have been coded by various phonological processes. The analysis of the loanwords helps to elucidate the origin of the kalamo, which may contain elements of a pre-existing Arabic pidgin. Their phonetic form shows that the Islamic migrants in Madagascar may indeed have come originally from the Arabian peninsula.