Abstract
The karyotypes of an adult female and male foetus of a ray, Mobula japonica, were observed following short-term tissue culture, and the cellular DNA content measured. The karyotype and cellular DNA content of Mobula japonica were determined as follows: 2n=66, M=26, SM=12 (female) and 11 (male), ST-A=28 (female) and 29 (male), FN=104 (female) and 103 (male), NAN=98, DNA=9.5pg/cell. A small difference recognized between the male and female karyotypes, in the shape and size of the one of the middle-sized chromosome pairs, may be related to sex determination in this species. The phyletic relationships of the order Myliobatiformes is discussed from karyological viewpoint.