The field vole (Microtus agrestis) is characterised by extremely large blocks of heterochromatin on both the X and Y chromosome. Some other Microtus also have blocks of heterochromatin on their sex chromosomes but not as extensive and always of independent origin from the heterochromatic expansion found in M. agrestis. Coupled with evidence of geographic variation in large heterochromatic blocks within other species (e.g. in the western hedgehog Erinaceus europaeus), it might be expected that field voles would show substantial variation in size and disposition of the sex chromosome heterochromatin. In fact, only minor variation has been described up to now. Those studies conducted previously were largely on field voles from central and northern Europe. Here, we describe the karyotype of field voles from Portugal, of interest because recent molecular studies have shown field voles from western Iberia to be a separate evolutionary unit that might be considered a cryptic species, distinct from populations further to the east. The two Portuguese field voles (one female, one male) that we examined also had essentially the same karyotype as seen in other field voles, including the giant sex chromosomes, but with small differences in the structure of the Y chromosome from that described previously. The finding that field voles throughout Europe show relatively little variation in their giant sex chromosomes is consistent with molecular data which suggest a recent origin for this complex of species/near-species.