During a preliminary limnological survey of a number of high altitude lochs in the Scottish Highlands a rich benthic bryophyte flora was noted in four of these with exceptionally clear water. Eight species of moss and seven species of liverwort were collected at depths ranging from 1 to 20 m. Some of these deep-water bryophyte communities were dense with luxurious growth covering many square metres. Although bryophytes have been collected from considerably greater depths in other parts of the world, the previous maximum depth from which they had been recorded in a British lake appears to be c. 12 m. Since a few of the mosses were detached and entangled in shoots of other species they had almost certainly been washed or blown into the lochs. However, the remainder were firmly attached to their rock or mud substratum and, although several are not typically aquatic and had almost certainly originated from terrestrial habitats, they had obviously adapted to the relatively stable deep-water environment. The lochs in which these bryophytes occur are covered by about 1 m of ice and varying depths of snow for up to 7 months of the year, thereby considerably reducing the amount of light penetrating the water. Ice scour and wave action have a marked influence in preventing widespread colonization of the littoral zone down to c. 1 m. We should like to thank Dr M. E. Newton and Mr M. O. Hill for confirming the identification of the mosses, and Mrs J. A. Paton for determining the liverworts.