We investigated morphological and allozyme variation and pollen viability in 22 populations of Nuphar japonica, N. pumila, and unidentified intermediate plants in Hokkaido, Japan, to assess hybridization between the two species and the potential existence of homoploid hybrid speciation. A phenogram based on cluster analysis using 15 morphological characters revealed three cluster groups. Groups 1 and 3 had distinctive morphological characteristics and corresponded to the published descriptions of N. japonica and N. pumila, respectively, whereas Group 2 showed intermediate values in most characteristics. In our allozyme study, many morphologically intermediate plants showed additive combinations of species-specific alleles from the two species. Pollen viability was significantly lower in percentage in intermediate populations than in the two species. Intermediate populations were found within the area of overlap between the ranges of the two species. On this evidence. we concluded that intermediate plants represent hybrids between the two species. Four hybrid populations showed different relationships between morphology and pollen viability. Hybrids in artificially disturbed populations had discontinuously intermediate morphology between their parental species and were completely fertile in some cases. It is probable that ecological selection has contributed to the discontinuous morphological patterns. As a result, homoploid hybrid speciation may be occurring in these Nuphar hybrids. (c) 2006 Elsevier B.V. All rights reserved.