From Peace Corps Wiki
The Aja are a group of people native to south-western Benin and south-eastern Togo. According to tradition , the Aja migrated to southern Benin in the 12th or 13th centuries from Tado on the Mono River and in the early 17th century, three brothers, Kokpon, Do-Aklin, and Te-Agdanlin, fought for the kingdom, dividing it amongst themselves. Kokpon took the thriving capital city of Great Ardra, Allada. Do-Aklin founded Abomey and Te-Agdanlin founded Little Ardra, also known as Ajatche and Porto Novo by Portuguese traders.
Those Aja living in Abomey mingled with the local tribe, thus creating a new people known as the Fon, or "Dahomey" ethnic group. This group is now the largest in Benin. Another source  claims the Aja were the rulers of Dahomey (Benin) until 1893, when the French conquered them. Currently, there are approximately 500,000 Ajas in an area straddling the border between Benin and Togo, thirty miles long and twenty miles wide.
The Aja speak a language known as Aja-Gbe, or simply 'Aja'; only 1-5% are literate in their native tongue. According to one source, voodoo originated with the Aja. There are three dialects: Tàgóbé (in Togo only), Dògóbè (in Benin only), and Hwègbè (in both countries). Many are trilingual, also speaking French and Fongbe, the lingua-franca of southern Benin, while Ewe is spoken as a second language by those Aja living in Togo.
Due to severe land shortages in the densely populated togolese-beninois border region mentioned above, many Aja have migrated in recent years, seeking arable land for subsistence farming or work in urban centers. There are a significant number of Aja living throughout the coastal region of Benin and Togo, southern Nigeria and Gabon. The urban centers of Cotonou, Lome, Lagos and Libreville all have significant Aja migrant populations