The June issue of the Journal of Helminthology comprises selected papers on ‘Parasites as biological tags and responses of their aquatic hosts to environmental change’. The use of parasites as biological tags or indicators for different aspects of the biology of their hosts is an important growth area in the field of practical parasitology. Two ways in which parasites are used for this purpose are as tags for investigating the stock structure of commercially important fish and aquatic invertebrates and as indicators of anthropogenic environmental change in aquatic habitats. Eight of the papers in this special issue were presented at the XI International Congress of Parasitology (ICOPAXI) held in Glasgow, Scotland, August 2006, in two sessions dedicated to the use of parasites as biological tags for marine fish. The aims of these sessions were to present the latest research in this field from around the world and to highlight new developments, notably the application of increasingly sophisticated multivariate statistical methods to parasite infection data and the use of molecular biology techniques in parasite identification. These sessions were truly international in their subject material, covering a wide geographical range including different parts of the North Atlantic, Southwest Atlantic, Eastern Pacific and the Mediterranean Sea, and a variety of fish species. In addition, this issue includes a morphological and ecological study of the parasites of a population of juvenile herring off the west coast of Sweden and two papers on the use of parasites and their hosts as indicators of anthropogenic change, also presented at ICOPAXI in a special session on aquatic parasitology and immunology sponsored by Fireflower Systems Limited, British Columbia, Canada.

I wish to express my sincere and grateful thanks to Dr Ken MacKenzie, School of Biological Sciences, University of Aberdeen for his exceptional and invaluable support in the publication of papers in this issue on parasites as biological tags.