The red patch located on the propodus of Cherax quadricarinatus males is a sexually dimorphic structure. This patch presents an enigma because it is soft and uncalcified, consisting of a thin red to whitish-orange membrane. It presents an impairment of the fighting capability of the males, which, like many other clawed crustaceans, use these appendages in intra- and interspecific aggressive interactions. Present knowledge on patch structure and colour, development and their endocrine control by the androgenic gland is reviewed. The function of the red patch as a possible sensory organ or social signal is discussed. The red patch may transmit information concerning the gender, size and quality of its owner. Several hypotheses concerning the signal function of the patch are suggested and evaluated in the light of our present knowledge of the social behaviour of C. quadicarinatus.