WE report here a new maternal effect mutation (nc, for no cleavage) in the Mexican axolotl, Ambystoma mexicanum. This mutation involves a defect in the assembly of microtubules in the fertilised egg. There is a large literature describing the molecular and biochemical events which accompany the in vitro self-assembly of microtubules; the nc mutant is the first known system with a genetic lesion in the regulation of these events in vivo. This mutant will provide a means to examine directly the control of microtubule assembly, and may also be useful in the identification of the events which trigger initiation of cleavage after activation of the egg at fertilisation.