Within the group of maternal effect genes necessary for the establishment of the dorsal-ventral pattern of the Drosophila embryo, the Toll gene mutates to give a singular variety of embryonic phenotypes. Lack of function alleles produce dorsalized embryos as a recessive maternal effect. Dominant gain of function alleles result in ventralized embryos. Other recessive alleles cause partial dorsalization or lateralization of the embryonic pattern. Gene dosage studies indicate that the dominant ventralized phenotype results from an altered activity of the Toll product. Complementation studies show specific trans interactions between copies of the Toll product. Double mutant phenotypes suggest that the products of several other dorsal-group genes regulate the activity of Toll.