The large copper butterfly, Lycaena dispar batavus, a subspecies of much conservation interest, is host-specific to the great water dock, Rumex hydrolapathum, but little is known of the relationship between herbivore and host in wild populations. This study investigated the distribution of both R. hydrolapathum plants and L. d. batavus eggs in four different habitat types within the Weerribben National Park, The Netherlands, during the summer of 1993. As expected, host plant distribution strongly influenced that of its herbivore. Further, both species had significantly aggregated distributions, and that of R. hydrolapathum was in agreement with the negative binomial model. Host plant selection for oviposition showed no significant relationship with physical plant characteristics, such as plant height and the number of leaves, and at the scale studied, habitat type was of only limited influence. The possible influence of the vegetational architecture surrounding host plants is discussed.