The genusTrichogramma includes numerous species which parasitize primarily the eggs of Lepidoptera worldwide. Thirteen European species were characterized biochemically to improve the systematic basis of this genus. Three enzyme systems, studied by electrophoresis, provided characters which discriminate among species. Nei distances were calculated to estimate the relationships among these species. Esterase bands proved to be a very reliable technique for discriminating among closely related species when morphological characters are not evident. The within and between population variability of isozyme patterns was greater than previously assumed forTrichogramma. The results allowed the affinity of some species to be reexamined.