Wessex Archaeology was commissioned by Scott Wilson Ltd to undertake an archaeological assessment of geophysical survey data prior to the extension of the quay at Saint Mary's Harbour. The geophysical data was comprised of sidescan sonar, sub-bottom profiler and single beam echosounder data and was reviewed in order to determine the archaeological potential of the survey area. The survey data was acquired between the 30th October and the 1st November 2004 from the survey vessel MV Enterprise by Titan Environmental Surveys Ltd. The survey area was located around the quay at St Mary's Harbour near Rat Island in the Isles of Scilly. The survey area was rectangular, 500m long and 600m wide orientated N-S between 89900E, 11200N and 90500E, 10700N (OSGB). On the basis of the geophysical data, the marine archaeological potential of the survey area can be summarised as follows: " Fifteen sidescan sonar anomalies were considered to be of low archaeological potential; " Four sidescan sonar anomalies were of apparent anthropogenic origin and should be investigated by an archaeological diving inspection prior to the development; " Two sidescan sonar anomalies located within the proposed development may be posts or some other anthropogenic structure; " The near surface geology of the area was composed of granite bedrock overlain by a thin layer of soft sediment which appears to contain no peat horizons or any other features of archaeological potential.