Steadman, which has gross assets of $3.3m, has its headquarters in Nairobi, Kenya. It was founded in 1984 and provides custom market research and media monitoring for African businesses and international clients.
The African research market is growing quickly and it achieved 15% growth in 2007 giving Aegis a commanding position.
Robert Lerwill, CEO of Aegis, said: "This acquisition is in line with our stated strategy of investing in high growth markets.
"It takes us to number one in Africa, strengthening our existing presence in North and Southern Africa and means that Synovate is now in a total of 63 markets globally."
The latest acquisition follows two recent purchases by Aegis. In June, it bought Russian digital agency Adwatch and search marketing agency Globlet.