Wallachia
Ottoman pressure traditionally forced members of several South Slavic communities to seek refuge in Wallachia - although under Ottoman rule as well, the latter was always subject to less requirements than regions to south of the Danube.
These groups are, however, hard to distinguish one from another in early Wallachian references, as the term "Serbs" is regularly applied to all Southern Slavs, no matter where they might have originated. This only changed in the 19th century, through a transition made clear by an official statistic of 1830, which reads "census of how many Serbs are resident here in the town of Ploieşti, all of them Bulgarians" (Giurescu, p. 269).
Serbs-proper probably constituted the vast majority of mercenary troops known as seimeni, given that their nucleus is attested to have been formed by "Serb seimeni" (as it was during their revolt in 1655), and that the rule of Prince Matei Basarab had witnessed the arrival of a large group of Serb refugees.
Read more about this topic: Serbs In Romania, History