Given the sympatric occurrence in some regions of Europe and the great epidemiological significance of D. reticulatus and D. marginatus species, we investigated the behaviour of these ticks during inter-specific and mono-specific host infestations.The investigations were conducted on rabbits at 20 ± 3 °C and humidity of 38 ± 1 %. The inter-specific infestations groups consisted of 20 females and ten males of D. marginatus and 20 females and ten males of D. reticulatus on each host, whereas mono-specific infestations involved 40 females and 20 males of each species. The investigations have demonstrated competition between the two tick species resulting in modification of the behaviour on the host and the feeding course in D. marginatus females by the presence of D. reticulatus. In the inter-specific group, D. marginatus females attached for a longer time (mean 2.74 ± 1.12 h) than in the mono-specific group (mean 1.24 ± 0.97 h) (p < 0.0001). The feeding period of these females was shorter (9.45 ± 1.30 days) than in the mono-specific group (13.15 ± 2.53 days) (p < 0.0001), but they exhibited a statistically significantly higher body weight in comparison with the females from the mono-specific infestation (p = 0.0155). In D. reticulatus females, no significant difference was found in the host attachment and feeding rates between the mono-specific and inter-specific groups.The differences in the behaviour of the females from both species during co-feeding reflect physiological adaptation to environmental conditions, which enables them to ingest blood and reproduce. During co-feeding of D. reticulatus and D. marginatus on the same host, two inter-specific systems with different physiological features are formed, which may influence the transmission of tick-borne pathogens.