Karyotypes and nuclear DNA contents of Pseudolabrus eoethinus and P. sieboldi were studied. Until a recent revision, the two species had been consid-ered a single species (Pseudolabrus japonicus), although two clearly distinct kary-otypes had been reported on several occasions. The karyotype of P. eoethinus was confirmed as 2M+2SM+44ST-A (2n=48, NF=52) and that of P. sieboldi, as 20M+8SM+14ST-A (2n=42, NF=70). Nuclear DNA content in the two Pseudolabrus species and 4 other labrid species from Japan was measured by flow cytometry, and provisionally expressed relative to that of a puffer fish, Canthigaster rivulata, used as an internal standard. The relative nuclear DNA content of P. eoethinus was 2.91±0.03 (n=5), that of P. sieboldi being 3.00±0.05 (n=6). Al-though the two values differed significantly (Mann-Whitney U test, P=0.01, Ucal=0.5), the very small difference between them (average value of latter 103% of former) together with karyotype comparisons suggested that both pericentric inver-sions and Robertsonian fusion had played a major role in the establishment of the more derived karyotype of P sieboldi.
