This study focuses on the effect of acidic water and aqueous aluminium on the monogenean ectoparasite Gyrodactylus salaris, infecting Atlantic salmon (Salmo salar) parr. G. salaris-infected salmon were exposed to various combinations of acidity and aluminium concentrations. The most pronounced effect was the elimination of parasites after 4 days when 202 μg Al/l was added to the water. The effect of aluminium was concentration dependent, but was relatively independent of pH (5·2, 5·6 and 5·9). At the lowest pH of 5·0 the effect of aluminium was enhanced. Acidic aluminium-poor water had no or minor effects on the G. salaris infections except at pH 5·0 where all parasites were eliminated within 9 days. The G. salaris populations increased exponentially in untreated control water. The results show for the first time that aqueous aluminium can, to a limited extent, have a positive effect on fish health. This study emphasizes that basic knowledge about abiotic environmental factors is of importance in order to understand the population dynamics, range extension and dispersal of ectoparasites such as G. salaris. Finally, our results suggest that aluminium treatment could form an effective disinfection method against ectoparasites in hatcheries and laboratories, as well as complementing the controversial rotenone treatments used against natural populations of G. salaris.