Behavioural observations and DNA fingerprinting were used to determine the relationship between male and female behaviours and levels of extra-pair paternity in the wheatear, Oenanthe oenanthe. Behavioural observations were consistent with the hypothesis that males attempted to ensure paternity by mate guarding, while pursuing extra-pair copulations (EPCs) primarily outside the fertile period of their pair female. The intensity of guarding varied with time of season and was greater at late nests. However, although males on territories with late nests also experienced high intrusion rates, the intensity of guarding was influenced more by the operational sex ratio (which was female skewed at early nests) than by intrusion rates per se. We suggest that early breeding males adopted a strategy of territorial defence to ensure paternity, as opposed to guarding their female directly (which late breeding males did), to capitalize on the increased opportunities to pursue EPCs in neighbouring territories. Females were less conspicuous than males in their pursuit of EPCs, were never seen off territory or observed to solicit extra-pair males directly, and rejected the majority of EPCs. The frequency of extra-pair paternity was 11% of 73 offspring, in 29% of 17 broods, and was not correlated with the intensity of guarding. Female cooperation appeared to be important for successful copulation, and extra-pair paternity is therefore likely to be a consequence of solicited, or at least accepted, EPCs. We discuss why females might have participated in EPCs. Copyright 1998 The Association for the Study of Animal Behaviour Copyright 1998 The Association for the Study of Animal Behaviour.