In the Australian red-claw crayfish Cherax quadricarinatus (vonMartens) (Decapoda, Parastacidae), a gonochoristic species, seven differentcombinations of intersex individuals (with both male and female genital openings)have been described. However, to date, the genetic basis for this phenomenon hasnot been investigated. This study was designed to test a simple chromosome-basedsex-determination model for C. quadricarinatus that assumes the male tobe the homogametic (ZZ) sex. According to our model, intersex individuals thatare functionally males are genetically females (WZ). Individual crosses wereperformed between intersex and female crayfish, with control crosses beingperformed between normal males and females. The control crosses yielded, in mostcases, the expected 1[ratio ]1 sex ratio in the F1 progeny. Crossesbetween intersex individuals and females yielded a 1[ratio ]3 (male[ratio ]female)sex ratio in most crosses. According to our hypothesis, one-third of the femalesproduced in a cross of a female with an intersex animal should be WW females. Thehypothesis was tested by crossing normal males with F1 females, whichwere progeny of intersex fathers. These crosses yielded almost 100% females, afinding that conforms to the above-suggested sex determination model for C.quadricarinatus and the female WZ genotype of intersex individuals.