The karyotype of male N. lugens has previously been reported as 2n-29 (14₁₁+X) giving an X O, X Y sex determining mechanism. However, this investigation revealed the alleged single X chromosome in the male consists of two elements of unequal length which separate reductionally in the first meiotic division and equa-tionally in the second. From this, and by comparison with other X O species it was determined that N. lugens has an X Y sex determining system with a chromosome number of 2n=30 (14₁₁+X Y) in the male.