Chromosome numbers are given for 1011 populations of 242 species, representing the full range of taxa (49 of the about 52 presently recognized genera) in the Neotropical Nymphalid butterfly subfamily Ithomiinae (prime movers for mimicry rings), including many additional geographical subspecies from 47 regions from Mexico and the Caribbean islands throughout all tropical South American countries to southern Brazil. Twelve Neotropical Danainae (in 3 genera), all but one with n=29-31, and the Australian Tellervo (n=32) served as sister groups for comparison. The numbers range near-continuously from n=5 to n=120 with modal values (33-84 counts) at n=12-18, and only 16 and 26 counts at the usual modal number of all butterfly groups, n=30-31. Superimposition of these changes in karyotype on a cladistic phylogeny of the subfamily indicates possible early halving of the complement to n about 14-15, followed by much variation in each genus and tribe. While at least 17 species in 15 genera show stable karyotypes over much of the Neotropics, at least 40 species show large geographical variation in number of chromosomes, rarely accompanied by any evidence for reduction in fertility or incipient speciation. The evolutionary opportunism of the members of this subfamily probably accompanies their known population biology and community ecology: they are common, shade-loving, highly gregarious (occurring in small multispecies "pockets" in deep forest) and often migratory as a community when the environment becomes unfavorable (too hot or dry).