Influence of Gyrodactylus sp. on the growth of Salmo gairdaeri is studied. The prevalence and density of Gyrodactylus spp. infection and details of fry, length and weight were recorded at a commercial table farm on the River Itchen, Hampshire, U.K. Monitoring on a wekly basis between September and December 1986. Two weeks after arrival of the fry at the farm, the first sample was taken (23rd September) and Gyrodactylus was found at 41% prevalence. The infection reached a peak of 100% prevalence on 8th October, (the parasite rate ranging from 1 to 8 per fry). The regression analysis showed that Gyrodactylus mucus infection was significantly correlated with fish condition factor (P