A preliminary survey was made of Loch Obisary, the largest brackish water loch in North Uist, Outer Hebrides. The loch has a single narrow connection to the sea, through which sea water enters at high tides. Salinity, temperature and depth measurements were taken throughout the loch and revealed the presence of a distinct halocline and thermocline in the north basin of the loch at about 4 m depth, neither of which were present in the south basin. The salinity of the water above the halocline in the north basin and throughout the south basin was 13 - 14 ppt whilst that below the halocline had a maximum of 28.5 ppt. Temperature decreased by approximately 1 degree Centigrade at the level of the halocline and rose again by approximately 0.5 degree Centigrade near the bottom of the loch. The flora and fauna were investigated both by diving and by shore collections. Two distinct communities of plants and animals were found in the loch: i) A 'freshwater ' marginal component consisting of freshwater euryhaline species and true brackish water species found above the halocline in the north basin and throughout the south basin of the loch. ii) A marine component consisting of marine euryhaline species numerically dominated by the red alga Phyllophora pseudoceranoides and the tunicates Ciona intestinalis and Ascidiella aspersa. Comparisons are made with other studies of brackish lochs and waters and it is concluded that Loch Obisary is an interesting and unusual example of this scarce habitat type.