Cytological studies of Simulium tuberosum s.l. from the type locality in northern Finland demonstrate the presence of three reproductively isolated species. One of these species, here designated FGIO, has not previously been described cytologically, whereas the remaining two cytospecies, AB and FGI, are both well known from North America and Norway. FGIO differs from its close relative, FGI, by one fixed inversion in IIS, one in IIL, and a unique sequence in the distal half of IIIL. Morphological characters of the larvae corroborate the cytological evidence for three distinct species. On the basis of morphological evidence, we suggest that AB is true S. tuberosum (Lundström) and FGI is S. vulgare Dorogostaisky, Rubtsov & Vlasenko. The identity of FGIO remains unknown, although it might be S. tumulosum Rubtsov. Morphological and chromosomal notes on S. tuberosum s.l. from Germany and England are also given; we suggest that the species known from England as S. tuberosum is actually unnamed. Overall, our study provides a blueprint for dealing with nominal taxa that are shown to be composites of sibling species.