Small mammals make up an important link in the food chain as many predator species feed on them. There are indications that small mammal populations in Europe are declining due to the intensification of agriculture. According to national legislation, farmers in Switzerland have to cultivate at least 7% of their land as ecological compensation areas and, thus, some alternative habitats that are possibly beneficial for small mammals have been created. In this study, we estimated the diversity and density of small mammals on two types of conventional farmland field types (artificial grassland and autumn-sown wheat) and three types of ecological compensation areas (wild-flower strips, herbaceous strips and low-intensity meadows) by use of capture-recapture in March, May and July 2003. The common vole Microtus arvalis was the most abundant and predominant species in all habitat types except in herbaceous strips, which harboured the highest diversity with six species caught. In March the density of small mammals was generally very low, but significantly higher in wild-flower (mainly due to common vole) and herbaceous strips than in the other habitat types. In wild-flower and herbaceous strips, densities increased strongly from March to May and in July. On autumn-sown wheat fields, a strong increase occurred only from May to July and was caused by common vole. On artificial grassland and low-intensity meadows, densities of small mammals (mainly common vole) increased only marginally with low-intensity meadows supporting slightly higher densities. Thus, habitats that were not mown each year supported the highest densities of small mammals. This demonstrates that ecological compensation areas, such as wild-flower and herbaceous strips, make up an important refuge for small mammals. They probably also have positive effects on populations of many predator species that depend on small mammals, particularly if a mosaic with mown surfaces is created.