Multi-locus monomorphism in microsatellites is practically non-existent, with one notable exception, the island fox (Urocyon littoralis dickeyi) population on San Nicolas island off the coast of southern California, having been called "the most monomorphic sexually reproducing animal population yet reported". Here, we present the unprecedented long-term monomorphism in relict populations of the highly endangered Parnassius apollo butterfly, which is protected by CITES and classified as "threatened" by the IUCN. The species is disjunctly distributed throughout the western Palaearctic and has occurred in several small remnant populations outside its main distribution area. We screened 78 individuals from 1 such relict area (Mosel valley, Germany) at 16 allozyme and 6 microsatellite loci with the latter known to be polymorphic in this species elsewhere. From the same area, we also genotyped 55 museum specimens sampled from 1895 to 1989 to compare historical and present levels of genetic diversity. However, none of all these temporal populations yielded any polymorphism. Thus, present and historical butterflies were completely monomorphic for the same fixed allele. This is the second study to report multi-locus monomorphism for microsatellites in an animal population and the first one to prove this monomorphism not to be the consequence of recent factors. Possible explanations for our results are a very low long-term effective population size and/or a strong historic bottleneck or founder event. Since the studied population has just recovered from a recent population breakdown (second half of twentieth century) and no signs of inbreeding depression have been detected, natural selection might have purged the population of weakly deleterious alleles, thus rendering it less susceptible to the usual negative corollaries of high levels of homozygosity and low effective population size.