The classical problem of decapod egg-membrane formation and egg attachment was studied in Carcinus maenas, Homarus vulgaris, Nephrops norvegicus and Astacus pallipes. It was found that no fundamental difference existed in the membranes of the above species, and that some of the current views on the physiological processes involved were incorrect. The present study indicated that no externally deposited membranes have been formed around the egg at any stage. The outer-most membrane really consists of three layers which are histochemically distinct from one another and cannot be equated with epicuticle. The outermost layer (layer 1) of this membrane is derived from the vitslline membrane, the second layer (layer 2) is formed through solidification of a fluid exuded from the egg at spawning, and the third layer (layer 3) is formed by highly PAS-positive substance from the egg. It is suggested that the formation of this ‘trichromatic membrane’ is initiated by fertilization.