In 1983 and 1984 an inventory was made of the edaphic mites and springtails in a six-year rotation, a three-year rotation and a three-year rotation in which the soil was disinfectedwith metamsodium after the potato crop was harvested. The aim was to find possible direct or indirect biological factors related to the yield loss observed in the three-year rotation potato crop. The test site was situated on a heavy marine clay soil. Samples were taken four times in the course of the growing season. The mesofauna was extracted by means of Macfadyen high-gradient extraction. The following depths were extracted: 0-2,5; 2.5-5; 7.5-10; 15-17.5 and 25-27. 5 cm. The results were examined by analytical and synthetical procedures. The mesofauna was much more abundant in cereals than in root and tuber crops. Most mites and springtails were found near the surface of the soil except in potato when it was preceded by a cereal crop that was plowed down. In 1983, a year with relatively heavy and prolonged rainfall, more mites were found in corresponding crops of the six-year rotation than of the three-year rotation. The short rotation reduced the abundance of Tectocepheus velatus, whereas the percentage of the Tullbergia krausbaueri group and the number of Heterostigmata were increased. In 1984, a year with colder and drier weather, more mites and springtails were found in the potato crop of the three-year rotation than in the potato crops of the six-year rotation. Correspondence analysis resulted in four axes that could be given a meaningful interpretation. Ordination of the samples and species along the first axis was mainly due to the presence of Tectocepheus velatus and was related with depth, rotation and climatic factors. The second axis showed a year-effect for the 1983 and 1984 potato crops. The species composition of the mesofauna in many ways reflected crop effects as shown by the ordination of the samples and species along axes 3 and 4. There was a suggestion of a possible contribution of the Acaridae to the yield loss. This has to be subjected to further study. The mesofauna was indicative of a low content of fresh decomposing organic matter and a compacted soil structure in the root and tuber crops. The high percentage (66%) of root and tuber crops is the most probable cause of the low abundance and species richness of the mesofauna in the three-year rotation. Lower yields in the three-year rotation potato crop coincided with the indication by the mesofauna of a more compact soil structure and less fresh decomposing organic matter, which was even more so in the disinfected three year rotation. Still, soil treatment with metamsodium partially reduced the yield depression.