Crustaceans are important hosts for a number of helminth parasites, and they are increasingly used as models for studying the physiology, ecology and evolution of parasite-host interactions. In ecological studies, this interaction is commonly described only in terms of prevalence and number of larvae per infected host. However, the volume of helminth parasites can vary greatly, and this variation can potentially give important insights into the nature of a parasite-host relationship. It may influence and be influenced, for example, by within-host competition, host size, growth, and life history. Here we present a simple method that allows rapid approximation of the absolute and relative volumes of cestode larvae within copepod hosts of various developmental stages (nauplii, copepodites and adults). The measurements are taken in vivo without much disturbance of the animals, i.e. the technique allows study of growth and development of the parasites in relation to that of their hosts. The principles of this technique can be adopted to other helminth parasites and other crustacean hosts. Using this method in the copepod Macrocyclops albidus infected with the cestode Schistocephalus solidus, we found that the relative parasite size (= `parasite index') ranged from 0.5% to 6.5% of host size 14 days after infection. It was greater in male than in female hosts. With increasing number of parasites per host, the total parasite volume increased while the mean volume of the individual parasites decreased. The magnitude of the observed parasite indices, the large variation that was found within a sample of 46 infected adult copepods, and the observed correlates suggest that this new index can indeed be an important measure of parasite success and its pathogenecity.