The distribution and the activity patterns of Dermacentor reticulatus in Wales were investigated. Two different methods of geographic variation analysis were employed. A morphometric study of variation in the scutal shape and base-pattern using discriminant analysis revealed a north-south cline running from Wales through Devon down to France. The similarity of the Welsh populations to one another suggest a common origin. A genetic analysis revealed little allozyme variation in the enzymes isocitrate dehydrogenase (ICD) and phosphoglucomutase (PGM), though a fast allele (ICD1) was 'found in some Welsh specimens and a slow allele (ICD') in a few French specimens. The seasonal activity, habitat and host associations of D. reticulatus was investigated at Morfa Harlech. Activity began in late August and lasted through to the following May with two peaks in activity, a marked peak in March-April and a more diffuse and reduced peak between September and November. There was no winter diapause. There were significant variations in activity from year to year and between sites (ungrazed, grazed and marsh) within the Morfa Harlech area. Macroclimatic temperature had a significant effect on activity at the ungrazed and grazed sites whereas photoperiod had a significant effect on activity at the marsh site. Microclimatic temperature had a significant effect on activity at the grazed and marsh sites and the microclimatic humidity also had a significant effect on activity at the grazed site. At Morfa Harlech ticks were found in a number of vegetation sub-communities within the dunes. Seasonal sex-ratio variation was also observed; in the autumn males appeared earlier than females, but females predominated for much of the year and were numericall y dominant. Adult ticks were found on Welsh black cattle but not on rabbits. Larvae and nymphs were found on four small mammal hosts at the marsh site, Clethrionomys glareolus, Apodemus sylvaticus, Sorex araneus and Sorex minutus. C. glareolus was the most important host species. Larval activity appeared to be over by mid-July and nymphal activity peaked during July. The more common and widespread Izodes ricinus showed a bi-modal pattern of activity with spring/early summer and autumn peaks and was active during the summer diapause of the rarer D. reticulatus and inactive during the period of winter activity in D. reticulatus. Experiments on the cold-hardiness in the two species showed no significant differences in their supercooling points (SCP's) and there were no marked differences in the SCP of upland and lowland populations of I. ricinus. The critical equilibrium humidity (CEH) of Welsh and French D. reticulatus (86-92% RH) is similar to that published for I. ricinus (86%).