The formation and development of monogenean communities on the gills of roach fry was followed in 1992 from early June to October (size range 9 to 47 mm). Roach fry (n=291) were sampled weekly from the small, humic River Rutajoki in central Finland. A further 209 roach fry were reared in a fish farm supplied by water from the river. Four Dactylogyrus species were found: D. nanus, D. crucifer, D. micracanthus and D. suecicus. Other species found on the gills were Gyrodactylus sp. and Paradiplozoon homoion. The first Dactylogyrus juvenile occurred on a 12 mm long fish fry in late June and the first adult (D. nanus) 1 week later in Tank 1. D. nanus was also the most common parasite in the river. Young fry had high numbers of Gyrodactylus sp. on the gills compared with adult roach in previous studies. Signs of site preference were found; D. nanus and D. micracanthus preferred the 2nd gill-arch and Dactylogyrus juveniles the outmost one. Abundances of monogenean infracommunities increased until the middle of August when, on average, 1·9 species and 3·7 worms per fry were noted and decreased then as the water temperatures fell. A lack of competition between species is suggested; for example, D. crucifer appeared on the gills at that time when abundance of D. nanus was at its highest. The roach fry hatched and reared in the fish farm revealed that larvae of the most common dactylogyrid species were able to disseminate and colonize over 120 m from the river to the farm. Influence of change colonization events was shown by the appearance and development of Gyrodactylus sp. population on roach fry in only 1 of the 2 tanks at the farm.