The objective of this study was to determine the number of macroinvertebrate samples or macrophyte surveys required to detect trends in biodiversity. In June 2006, 2007 and 2008 macroinvertebrate samples were collected from 25 ditches in a Dutch fen area. Ephemeroptera, Trichoptera, Coleoptera and Gastropoda were collected from the samples and identified. In 2006, 2007 and 2008, between May and August, monitoring of macrophytes was carried out in the same ditches. The ecological status of all ditches was classified as good. Spatial variability was optimized by sampling ditches with a maximized difference in abiotic and biotic conditions. The number of samples required to detect trends in biodiversity, expressed as the number of rare species and the number of indicator species, was determined. Incentive for this research was the European Habitat Directive with its objective to preserve, protect and improve the quality of the environment in terms of biodiversity. For the purpose of the Habitat Directive special areas of conservation (SACs) have been appointed in each member state. At present, hardly any monitoring of aquatic ecosystems in SACs is carried out in the Netherlands. To gain insight in trends in environmental quality in SACs knowledge on the monitoring requirements for the detection of trends is required.