The spatial organization and relationships of Apodemus sylvaticus and A. flavicollis in mixed deciduous woodland were examined using live-trapping techniques. Each population was aggregated and contagion was greatest during early summer. Aggregation in A sylvaticus varied inversely with density so that at high densities this species was randomly dispersed. In A. flavicollis aggregation increased with increasing density. This interspecific difference may result from differing annual population cycles. Adult males, adult females and juveniles had different patterns of dispersion and association in A. sylvaticus and A. flavicollis suggesting differences in their social behaviour. Intraspecific associations, particularly among males and between males and females, made a major contribution to overall contagion in the populations. A. sylvaticus and A. flavicollis were segregated throughout the year except for late summer and they were also prevalent in different cover types. The behaviour which determined the spatial segregation of the species may be acquired since juvenile A. sylvaticus were associated more frequently with A. flavicollis and juvenile A. flavicollis were associated less frequently with A. sylvaticus than their respective adult conspecifics. Comparing sympatric with allopatric populations of A. sylvaticus, and observing inter-specific behaviour, suggest that the spatial segregation in sympatry results from A. sylvaticus avoiding the competitively superior A. flavicollis.