This study deals with the ecology of ciliated protozoa in the benthos of a small (10 hectares), eutrophic loch (OS ref. 808 966). Regular sampling of the benthos and overlying water at three markedly different sites over a two-year period was complemented with laboratory experiments on individual ciliate species. In part 1 of the thesis, variation in the complete ciliate population (temporal and vertical distribution of numbers and biomass) is analysed by single and multiple regression on the other factors recorded (including redox potential, sulphide ion activity, pH, oxygen flux, nematodes, bacteria, chlorophyll a and phaeophytin, sediment density and organic carbon, temperature, daylength and various aspects of water chemistry). Seasonal cycles were recorded for ciliate number, biomass and pattern and extent of depth penetration. The most prominent seasonal characteristic was the accumulation of ciliates at the sediment surface during the summer (up to 83000 cm-2, > 99% being in the top 1 cm of sediment). This has been explained in terms of the summer increase in benthic productivity increasing the available food sources for ciliates at the sediment surface, and upwards migration of the redox potential discontinuity forcing the ciliate population into the oxidised microzone. In the second part, the distribution and abundance of individual species has been described. Species such as Cyclidium glaucoma and Aspidisca costata occurred on almost every sampling occasion. Large species such as Loxodes striatus, Spirostomum teres and Frontonia leucas were more frequent and abundant during the summer. Species with known or suspected tolerance of anoxia and reducing conditions, such as Caenomorpha spp., Mylestoma uncinatum and Epalxella spp., were more common in the sub-surface sediment. Although 91 species were identified during the whole sampling programme, the maximum number distinguished on any one occasion was 25. Computer-assisted association analyses identified groups of similar species and similar occasions. Species-groups composed predominantly of large ciliates were defined for each of the three sites. Occasions were sorted into seasonal groups, the species-groups composed of large ciliates accounting for most of the coincidence with the summer and autumn occasion-groups. The factors controlling seasonal variation are discussed. Part 3 is concerned with the energetics of the complete ciliate population and its constituent size classes. While estimates of production were high, (345 149 and 40 J cm-2 year-1 at the three sites) respiratory energy losses were relatively low (18, 11 and 4 J cm-2 year-1 at the three sites with ciliates accounting for a maximum of 3.5% of total benthic respiration). Seasonal variation was detected in population net production efficiencies (range 63 - 98%), the lower values being more common in the winter, The energetic efficiencies of the ciliate population are considered with reference to data in the literature for higher organisms. The contribution made by ciliates to energy flow through the benthos is discussed.