A Byzantine harbour (Theodosian harbour) has been uncovered during excavations at Yenikapi, with a stratigraphic sequence spanning the past 7000 BP. In the marine part of the sedimentary sequence, a high-energy deposit has been interpreted as being of tsunami origin and related to the earthquake of 557 AD. This paper presents a bio-sedimentological analysis of this facies. The unit is characterised by coarse sands and gravels containing reworked material such as woods, bones, marble blocks, amphora fragments, ceramics, coins, shells and plant remains. The thickness of the facies varies between 10 and 100 cm. The sediment matrix is poorly sorted with skewness values indicative of a sub-tidal fine-sand environment. Many of the marine taxa have been reworked and diverse ecological assemblages are represented (lagoonal, coastal and open marine species). This unit is divided into three facies consistent with different phases of the tsunami drowning and water retreat. The basal facies corresponds to two tsunami wave trains, and the upper facies indicates the backwash flow. (C) 2011 Elsevier Ltd and INQUA. All rights reserved.