1. The St Kilda wren (Troglodytes t. hirtensis) is confined to the St Kilda group of islands, some 50 miles out in the Atlantic. It was first described by Seebohm in 1884. At once collectors rushed for specimens, and in 1888 it was in danger of extinction. The Wild Birds Protection (St Kilda) Act, 1904 was passed to protect this bird, and subsequent increase occurred. 2. In 1931 a census was made on all the islands of the St Kilda group, and 68 nesting pairs were found in all; 45 on Hirta, 11 on Dún, 9 on Soay and 3 on Boreray. 3. These 68 pairs were nesting in the three habitats described; puffin slopes 37, steep cliffs 19, and buildings 12 (all these in the village on Hirta). 82 per cent. on the cliffs, but none nest on the large areas of moorland, hillsides, etc. The optimum density along the cliffs is about 150-300 yards between nests, in the village 70-200 yards (average 90). 4. The average size of a territory in the village area is 3000-5000 square yards. 5. In one pair especially studied the territory was found to be composed of a number of subterritories, small areas in which food was taken. These food territories constituted only 2.6 per cent. of the whole territory, and 85 per cent. of the food for the young was obtained in 1 per cent. of the whole. 6. Food territories are further divided into sex territories, each parent tending to feed in exclusive food patches not used by the other. 7. Relations with other species are more or less negative. 8. Food is arthropodan, small beetles, flies, centipedes, earwigs, geometrid and other larvae, Thysanura and spiders in considerable quantities. Small seeds and vegetable tissues are also recorded. 9. Young were in the nest, hatching to flying, in August. There was no sign of an earlier brood. 10. Local movements to non-nesting areas take place in the autumn, while migration probably occurs. 11. Natural enemies are absent, with the one exception of man. The species will be best preserved by being left entirely alone.