The Enchytraeidae (Annelida, Oligochaeta) are small whitish worms especially prevalent in moorland and coniferous forest soils. They are less abundant but show varying densities in agricultural soils. They are important in soil decomposition processes and soil structure formation. This thesis reports the results of a study into the population sizes of the enchytraeids in agricultural plots at Craibstone Farm, five miles west of Aberdeen, Scotland, and the reasons for the variations in population size. It also describes the effects of the enchytraeids on soil structure, as determined from careful study of soil thin sections by light microscope and image analysis. The agricultural plots at Craibstone Farm grew an eight-year crop rotation at seven different pH levels. The average enchytraeid population size was found to be 3,5000 m^-2 in the first year and 11,000 m^-2 in the second. Populations tended to be larger at lower pH (4.2 to 5.2) and under pasture rather than annual crops. Fertilizer was seen to reduce numbers in the short-term but increases were seen in the long-term. In nearby grassland and mixed woodland, populations were considerably larger due to higher soil organic matter and moisture contents. At all sites the enchytraeids concentrated in the top 5cm of the soil, again correlating with moisture content. During the two years of sampling populations were seen to be smallest in late summer, coinciding with low rainfall. Soil thin sections showed that at low pH enchytraeids produced abundant granular faecal pellets which eventually fused to form a soil matrix. At higher pH (5.2 and above) small clusters of faecal pellets were more commonly seen partially or totally infilling soil channels and earthworm vermiform fragments were seen to be broken down into smaller granules.