This study established that in an open rough-grazing habitat in Ireland a large proportion of female ticks mated in the vegetation rather than on the host, confirming observations made by other workers in scrub and forest habitats in Switzerland and Russia. It was found that many ticks active from October onwards had already mated even though they were moulted and apparently derived from the spring population. This activity was at odds with the view that such ticks are prevented from becoming active in autumn by a behavioural diapause. Laboratory and plot experiments involving observations on questing activity and feeding showed that the behavioural diapause of females was abolished by exposure to males. Male ticks did not show a behavioural diapause. In areas of high tick density this mechanism could result in the transfer of ticks from spring to autumn populations and, since the mortality rate of the autumn population is higher, could also contribute to density-dependent population regulation.