The main drivers of Svalbard reindeer population dynamics are likely to be limited food resources, periods of harsh winter weather and their abundant parasitic nematode infections. To show parasite demographic impact requires three approaches: field observation to document life history and abundances of parasites/hosts; manipulation of infection to quantify the effect of parasite intensity on host fitness; appropriate population models of density-dependent transmission. We monitored the reindeer population and intensity of parasites in culled reindeer, and treated a randomly selected reindeer group with an anthelmintic, comparing their fitness with a control group. The two main nematode species differed in life histories. Ostertagia gruehneri infected reindeer over the summer. Marshallagia marshalli transmission was limited to the harsh arctic winter. This implies that our treatment only affected O. gruehneri and showed that reindeer fecundity depends on intensity of O. gruehneri infection, which varied between years and was positively related to host population size. Modelling this interaction suggested a role for O. gruehneri in reindeer population regulation. More experiments with a delayed anthelmintic treatment, designed to manipulate M. marshalli numbers over the winter, provided little evidence of this parasite’s impact on host population dynamics.