Gyrdicotylus gallieni is a viviparous gyrodactylid monogenean that infects the buccal cavity and pharynx oi Xenopus laevis laevis. Offspring directly recolonize the same host as their parent and transmission is carried out exclusively by adult worms which are capable of leech-like stepping locomotion. The infrapopulation dynamics (at 20 °C) of this species were studied by experimental exposure to single worms of previously uninfected hosts (which were dissected at various time intervals post-infection). Infrapopulations increased exponentially until 50 days post-infection (p.i.), during which time the intrinsic rate of increase was estimated by regression as 0·065/parasite/day. After this, infrapopulation sizes showed greater variability, with established infections becoming extinct 2–5 months p.i.: elimination was probably due to someform of host reaction, as parasite lineages were maintained for up to 10 months by transfers to successive uninfected hosts.The development of infrapopulations is similar to that reported for other viviparous gyrodactylids parasitic on the bodysurface of teleosts. However, the intrinsic rate of increase was comparatively slow in G. gallieni and the duration of infection in isolated hosts prolonged. Slow-growing infrapopulations may elicit a host response more slowly and extend the period during which dispersal to a new host or host population can occur. This might be important in G. gallieni for which the internal site of infection could limit the transmission rate, as worm migration from the oral cavity or accidental detachment and expulsion is necessary for host-host transfer to occur. Exposure of wild caught X. l. laevis of unknown infectionhistory to 1 (n = 33) or 10 (n = 10) worms produced only 2 established infrapopulations (in both cases hosts initially infected by single worms), compared with establishment of over 60% in naive hosts infected with single worms: this suggests that host resistance may be an important factor in the population dynamics of G. gallieni.
