Karyotypes of nine Phalaenopsis species and the closely related Doritis pulcherrima were compared based on Feulgen- and DAPI-stained somatic metaphase chromosomes prepared from root tips. All species had the same chromosome number<$>(2n\equals 2x\equals 38)<$>, but their karyotypes differed markedly in absolute chromosome size, relative chromosome size, and the position of the centromere. Both genome size and the amount of constitutive heterochromatin (CH) varied widely among the species studied, and there was a positive correlation between these two parameters. The distribution of CH in the genomes was non-random: one or both arms of long chromosomes usually possessed large blocks of CH, while the small chromosomes contained little or no CH. DAPI-staining revealed that most CH regions are rich in AT base pairs. We suggest that differential accumulation of CH is a major cause for karyotype variation in Phalaenopsis orchids.