Current control methods rely on immersing affected fish in a parasiticide. In the UK only dichlorvos (Aquagard, Ciba-Geigy) is authorized for this purpose and is licensed as a veterinary medicine under the Medicines Act 1968. In practice fish are enclosed in a known volume of water, the chemical added in a quantity calculated to expose the fish to 1 ppm dichlorvos and the chemical is released after 1 h (Wootten et al. 1982). Dichlorvos is only active against mobile preadult and adult lice, and chalimus larvae appear to be unaffected. The decision to treat a population of fish may be determined simply on observation of a ‘significant’ burden or may be based on continuous monitoring of the numbers and population structure of the parasite. The detection of a predetermined number of mobile lice on a sample of fish will then trigger treatment. Knowledge of the number of chalimus present and of the speed of succession of generations gives advance warning for future treatments.