Fifteen spatially separated rural cat populations were sampled during 2007 in North-Eastern France near the city of Nancy (Fig. 1, black rectangles) . The distance separating neighboring cat Figure 1 . The study area. We identified three metapopulations (grey areas). Studied cat populations are represented with black rectangles and solid arrows represent the suspected interactions between the studied populations. Some unstudied populations may interact with the studied ones (dashed arrows) and are represented by white rectangles. doi:10.1371/journal.pone.0007377.g001 populations varied from 1.2 to 4 km. The study zone covers a territory of approximately 250 km 2 . In order to delimit the study area, we considered the geographical characteristics that might limit movements between the studied cat populations and those outside of the studied area. The spatial organisation of geographical barriers suggests that the populations may be organized into three distinct metapopulations, with rare contacts between cats of different metapopulations (Fig. 1, grey areas) . At a finer scale, behavioral observations reveal that males can disperse between populations along roads. By adopting a basic assumption that populations are considered connected when they are not too distant from each other (i.e. less than 2 km) and are connected by roads, we propose a connection network between the different populations (see Fig. 1 , solid arrows). Unfortunately, it was not possible to establish a fully isolated perimeter (at least in relation to spread of FIV), so some populations of the study area are in fact connected to unstudied populations ( Fig. 1 , white rectangles -the connections to the study populations are represented by dashed arrows). In particular, Saulxure-Les-Vannes is connected to several study populations. However, it was not considered for sampling because of potential bias due to a cat culling programme there. The village of Sepvigny, which is connected to Champougny, could not be sampled for technical reasons.