The tachinid fly Compsilura concinnata (Meigen) was introduced to North America in 1906 for the biocontrol of the forest defoliators Lymantria dispar (L.) and Euproctis chrysorrhea (L.) when these caterpillars were defoliating large areas of eastern Massachusetts. Most of the flies released, however, were the progeny of adults emerging from E. chrysorrhea caterpiliars collected in Eastern Europe. Although this tachinid is a generalist parasitoid, in Europe it is considered an important specialist of pestiferous species based on the level of its recovery there. Also, as one of the first insects established against forest defoliators in the Northeast, C. concinnata is in a premier position of providing information about colonization that increases the understanding in population genetics of how founder effects and dispersal influence the establishment of exotic insect populations. Initially in this study, a historical analysis was conducted to prepare a chronology of release of C. concinnata into North America and delimit its distribution. The introduction and release of C. concinnata was relatively restricted as they were primarily reared from one host E. chrysorrhea collected in Eastern Europe. More importantly, the extent to which this fly moved ahead of successive releases suggested there may be a historical basis to expect little differentiation among their nearctic populations. The literature on population genetics and use of electrophoretic analyses were surveyed to establish their use with C. concinnata in North America. Enzyme electrophoresis has been widely used for delimiting populations of non-native parasitoids and tracking their movement into new hosts and localities. To determine the suitability of enzyme electrophoresis for a population genetic analysis of C. concinnata, selected isozymes were identified and cataloged in an established laboratory strain. The analysis of isoenzyme patterns for measuring the relatedness among and between individuals, is useful in molecular studies of population genetics provided these patterns are heritable. To determine the heritablility of isozymes in C. concinnata, an electrophoretic analysis was conducted using both the parents and adult progeny of observed matings. The genetics of some C. concinnata isoenzymes followed simple patterns of Mendlian inheritance and were allozymic. The population genetic structure of C. concinnata in North America was examined with trap-hosts placed throughout the Northeast. The flies reared out were sampled with protein electrophoresis. There were slight differences in the allozymes and allele frequencies of C. concinnata and in comparison with other Diptera, the variation among Northeastern populations was relatively low. The variability among Northeastern populations of this parasitoid reflect limited genotypes in the founding populations. Over time new differences in protein loci between geographically separated populations probably arise through selection or drift than through non-lethal mutations. The time needed for protein coding loci between distantly spaced populations of introduced insects to diverge sufficiently for them to be detectable has not been determined. Estimating the time needed for allozymes of introduced insects to reject such differentiation is useful in understanding the evolution of biotypes, but is not a trivial process. Historical research in combination with analyses of the variability in allele frequencies among populations can be useful in determining the extent of divergence in non-native species that may lead toward new biologically adapted forms.