Chromosome counts in meiotic metaphase plates in the gonads of 67 of the probable 68 species of mimetic neotropical heliconian butterflies (Nymphalidae), representing 1524 individuals in 617 subspecies and geographically separate populations from southern Texas to northern Argentina, revealed a consistent haploid number of n = 21 in the genus Heliconius (except for the most advanced species with n = 33, 37, 56, and 60) and n = 31 in the more primitive genera (Eueides, Dryas, Dryadula, Agraulis, and Dione), with a transitional genus (Neruda) showing three species with n = 28-32, 21-22 + 5-10 "microchromosomes", and 20-22 + 1-5 "microchromosomes". The genus Laparus, with a single polymorphic species doris, probably an offshoot of early Heliconius, shows wide karyotypic variation (n = 20-30, 38) sometimes even within a single individual. The two most primitive genera also show much variation: Podotricha has two species with n = 9 and n = 26-29; and Philaethria shows many phenotypically similar species, two with n = 29 and a still uncertain number (at least 3) with n = 88 (most common), 67-72 (most widespread), 62 (very restricted geographically), 52, 21, and 12. Several interspecific hybrids (Heliconius cydno x H. melpomene) showed normal chromosome pairing, while deficient pairing was seen in intersubspecific hybrids in Eueides tales and Heliconius sara. The importance of these results in the evolutionary study of polytypic tropical species is discussed.