AbstractPrevious studies established the existence of morphologically highly similar amphiatlantic populations of the predominantly interstitial genusMicrophthatmus(European coastline ‐ from the German Bay to the Bay of Biscaya; American coastline ‐ from Maine to Massachusetts and at the coast of North Carolina). Originally these three populations were seen as belonging to a single species. By using a broad spectrum of different methodologies for character investigations (especially through the use of electron microscopy) three, distinct species can now be distinguished:M. listensisWestheide 1967,M. nahantensisn. sp. andM. carolinensisn. sp. While habitat structure and external morphology ‐ generally important in polychaete systematics ‐ were found to be rather similar between the three species, this study uncovered distinct differences between the species in their internal organization. Such sharp differences could even be followed down to the ultrastructure of single cells (e. g. copulatory stylets, position and number of ovaries, sperm and secretory granules of male accessory glands). Within population character variability is significantly lower in several features ofM. carolinensisthan in the same characters of the other two species. Similarities as well as differences have been noted in behavioural features and population dynamics between the