Evidence on adverse selection in slave markets remains inconclusive. A necessary prerequisite is that buyers and sellers have different information. We study informational asymmetry on the slave markets through notarial acts on public slave auctions in Mauritius between 1825 and 1835, involving 4,286 slaves. In addition to slave characteristics, the acts document the identities of buyers and sellers. We use this information to determine whether the buyer of a slave was related (e.g. a relative or a spouse) to the original slave owner, and thus most likely better--informed than other bidders. Auction--theoretic models predict that bidding should be more aggressive when informed bidders are present in open-bid, ascending auctions, such as slave auctions. By proxying informed bidders by related bidders, our results consistently indicate that this is the case, pointing toward the presence of information asymmetry in the market for slaves in Mauritius.