Here, for the first time, we describe the karyotype of <i>Myiopsitta monachus</i> (Psittacidae, Arini). We found 2n = 48, corresponding to the lowest diploid number observed in Neotropical Psittaciformes so far, with an uncommonly large W chromosome homomorphic to the Z. In order to better understand the evolution of the sex chromosomes in this species, we applied several molecular cytogenetic approaches, including C-banding, FISH mapping of repetitive DNAs (several microsatellite repeats), and whole-chromosome painting on metaphases of<i> M. monachus</i>. For comparison, another species belonging to the same tribe but with a smaller W chromosome (A. aestiva) was also analyzed. The results show that the constitutive heterochromatin has a very diverse distribution pattern in these species revealing heterochromatic blocks in the centromeric region of all chromosomes and in most of the length of the W chromosome in <i>A. aestiva</i>, while in <i>M. monachus</i> they were found in interstitial and telomeric regions. Concerning the microsatellites, only the sequence (CG)_n produced signals on the W chromosome of<i> A</i>.<i> aestiva</i>, in the distal region of both arms. However, in <i>M. monachus</i>, (CAA)_n, (CAG)_n, and (CG)_n probes were accumulated on the W chromosome, and, in addition, the sequence (CAG)_n also hybridized to heterochromatic regions in macrochromosomes, as well as in microchromosomes. Based on these results, we suggest that the increase in length of the W chromosome in <i>M. monachus</i> is due to the amplification of repetitive elements, which highlights their significant role in the evolutionary process of sex chromosome differentiation.