Allozyme and morphological differentiation patterns within the Carex flava complex in Fennoscandia were described by means of multivariate analyses. As a result of these studies, and considering also data on ecology, distribution and hybridization, the following taxa were recognized: C. flava with no infraspecific taxa, C. lepidocarpa with two subspecies, ssp. lepidocarpa and ssp. jemtlandica, C. demissa with no infraspecific taxa, and C oederi with three varieties, var. oederi, var. bergrothii, and var. pulchella. Carex lepidocarpa ssp. jemtlandica was allozymically divergent from ssp. lepidocarpa, indicating a long period of isolation-of the two subspecies. A poor morphological separation in some localities is apparently due to secondary contacts in connection with recolonization of Fennoscandia after the last ice age, resulting in the formation of fertile hybrids and back-crosses. The subdivision of C. oederi into three varieties is supported by ecological data and it is suggested that they have originated by parallel ecotype formation under similar selective regimes in different areas. However, the varietal limits are not-supported by differentiation in neutral (allozyme) characters. A phylogenetic hypothesis was constructed by means of neighbour-joining based allozyme data and it was indicated that C demissa may be more closely related to C lepidocarpa than to C oederi. Previous treatments that have reduced the C. flava complex to one or two species may have been based on inappropriate species concepts, too coarse methods of analysis of morphological data, ignorance of patterns of co-occurrence and hybridization, or a combination of these factors.