The recovery and analysis of a novel class of zeste mutations is described. z and z do not require two w genes for their expression unless the z gene is present. Analysis of genetic interactions among zeste alleles proved that z mutations are strong antagonists of the z gene product. z is readily reverted by X-rays or by ethylmethane sulphonate to a range of intermediate zeste alleles and thus it is considered to be an insertional mutation. We suggest that z and op mutations arose as a result of insertions in a presumptive control region, and two alternative models for the structure of the zeste locus are evaluated.