Ostracodes were employed for paleoenvironmental reconstruction at the Tower of London; the moat contains aquatic sediment dating back at least to the time of Edward I (r. 1272 -1307). Cores, penetrating modern and medieval sediments, yielded twenty-three species and a few foraminifera. In the western part, ostracodes in the medieval sediments indicate shallow water (~3m or less), salinities from 3 to 16.5 parts per thousand, and a mean annual air temperature (MAT) of about 7 degrees C. In the north part of the moat, all intervals (medieval) were also deposited in shallow water. Analyses of ostracodes indicate brackish water salinities and a MAT similar to that of the west side. Medieval sediments from the south portion contained species of Cytherura, Leptocythere, Machaerina, and Paradoxostoma, indicating salinties were at least mesohaline and probably close to fully marine (30 parts per thousand). This is in contrast to the lower values on the west and north sides, but not suprising in that the south side was next to the River Thames which had significant salinity levels upstream to the old London Bridge. Thus, the waters in the moat at the Tower of London in medieval times were mesohaline to fully marine, shallow (~3m or less), and subject to MAT's of 6 to 8 degrees C, in contrast to the current MAT of about 11 degrees C in central London.