Callithrix kuhlii occurs between the Rio de Contas and Rio Jequitinhonha in southern Bahia, just entering the north-easternmost tip of the state of Minas Gerais (Santos et al. 1987; Rylands et al. 1988). The western boundary is not well known, but undoubtedly defined by the inland limits of the Atlantic coastal forest. I. B. Santos (in Rylands et al. 1988) observed hybrids of C. penicillata and C. kuhlii in the region of Almenara, Minas Gerais, left bank of the Rio Jequitinhonha (16°41’S, 40°51’W). Its range is largely coincident with that of the Golden-headed Lion Leontopithecus chrysomelas. These two callitrichids are broadly sympatric.

Surveys in 1986/1987 by Oliver and Santos (1991) demonstrated the presence of forms intermediate in appearance between C. kuhlii and C. penicillata north from the Rio de Contas, along the coast up to the regions of Valença and Nazaré, just south of the city of Salvador (Mittermeier et al. 1988). Individuals observed by Rylands near to Nazaré, just south of the city of Salvador lacked the white frontal blaze, and, although retaining the pale cheek patches typical of kuhlii, were paler grey. A photograph of the marmoset from Valença, Bahia, north of the Rio de Contas, is shown in Mittermeier et al. (1988, p.19). The variation in pelage colour of the marmosets in this region is considerable, but Coimbra-Filho et al. (1991/1992), showed that true C. kuhlii extended north through coastal Bahia into the state of Sergipe as far as the Rio São Francisco in the recent past. The present-day confusion has arisen from the widespread forest destruction, most marked and nearly total in Sergipe, and the introductions and invasions of C. jacchus and C. penicillata.