Young-of-the-year (YOY) brook trout (Salvelinus fontinalis) were collected at 2 wk intervals (15 April to 4 June 2012) after emergence from redds and occurrence of the ectoparasite Gyrodactylus colemanensis (Monogenea) recorded. Emergent YOY were not infected. Infected trout (14%) first appeared 15 May, with host age estimated to be 4 wk or less post-emergence. Prevalence and intensity increased over the study period and reached, respectively, 93% and 8.2 ± 6.1 by 4 June. Prevalence and intensity of infection was also determined for older cohorts (1+, >1+) on 6 June, with G. colemanensis occurring on representatives of all age groups. The number of hosts in each age group was estimated by mark/recapture electrofishing over a 100 m reach of the stream. Combining estimated host numbers by age group with parasite prevalence and intensity data from those groups allowed calculation of parasite total standing crop. The results indicate that, by late spring, the new host recruits already carried 64% of the parasite population in the stream, serving to illustrate the remarkable efficiency of transmission by this viviparous monogenean in a temperate stream system.