The Common Marmoset occurs in the scrub forest (forest patches in dry caatinga thorn scrub) and Atlantic forest of north-eastern Brazil, in the states of Alagoas, Pernambuco, Paraíba, Rio Grande do Norte, Ceará, and Piauí, Maranhão, Bahia, and possibly north-eastern Tocantins, originally extending south as far as the Rio São Francisco and its west (left) bank tributary the Rio Grande (about 11º30’S). Hershkovitz (1977) indicated that it also probably extends north-west into the state of Maranhão, to the left bank of the Rio Parnaíba and the Serra do Valentim (Hershkovitz 1977). Hershkovitz (1977) extended the distribution no further west than the middle reaches of the Rio Grande (left bank) and the upper Rio Parnaíba (right bank), with a lacuna between these points and the Rio Tocantins. Silva Jr. (1999) reported on localities in Maranhão and Piauí marking the north-western limit to its range, and determined that, as Hershkovitz (1977) had indicated, it extends to the left bank of the Rio Parnaíba, but there is a lack of information concering its occurrence or otherwise west from there into the basin of the Rio Itapecuru (Sillva Jr. 1999; unpubl. data, 2008). The Black-handed Tamarin, Saguinus niger, occurs to the west, but the easternmost locallities are in the interfluvium of the rios Mearim and Itapecuru (J. S. Silva Jr., unpubl. data, 2008). Flesher (2001) recorded C. jacchus in the Serra das Mangabeiras at the headwaters of the Rio Parnaíba in Piauí, approximately 10ºS, 46ºW. South of the Serra da Mangabeiras, it is possible that the Serra Geral de Goiás marks the divide with C. penicillata to the west. It has spread into numerous other regions as a result of introductions outside of its original range, south of the Rio São Francisco, accompanying the destruction and degradation of the Atlantic coastal forest and its associated ecosystems (Coimbra-Filho and Câmara 1996). Introduced and recent populations include those in the state of Sergipe and the north and north-east of Bahia, including the ‘Recóncavo da Bahia’ (Alonso et al. 1987), the state of Rio de Janeiro in south-east Brazil (Coimbra-Filho, 1984; Ruiz-Miranda et al. 2000), the Ilha de Santa Catarina in southern Brazil (Santos et al. 2005) and they are also reported to have established themselves in Buenos Aires. Alonso et al. (1987) indicated that the Recóncavo da Bahia shows a relatively narrow zone of mixing between Callithrix penicillata and C. jacchus. However, Coimbra-Filho et al. (1991/1992; Coimbra-Filho and Câmara 1996) have shown that this region was originally forested, and argued that the destruction of the natural vegetation over vast areas since the European discovery of Brazil in 1500, along with frequent and repeated introductions, certainly of C. jacchus but probably also of C. penicillata, has resulted in a confused picture of hybrids between these species and between C. penicillata and C. kuhlii (see Coimbra-Filho et al. 1993). They argued that pure C. kuhlii was the original form occurring there.