The fs(1)gastrulation defective (dg) locus is one of the dorsal-group genes of Drosophila. Maternal expression of this gene is required for gastrulation movements and the differentiation of structures along the embryonic dorso-ventral axis. Twelve alleles of gd displayed a complex pattern of complementation, suggesting a direct interaction between subunits of a multimeric protein. Essential expression of the gd locus was strictly maternal with no zygotic contribution by the paternally derived allele. Clonal analysis revealed that expression of the gd locus was required in the germ line and that extreme dorsalization represented the null gd phenotype. Temperature-sensitive (ts) alleles displayed a ts period that included the last 4-5 hr of oogenesis and the first 1.5-2 hr of embryogenesis. Eggs from one ts allelic combination displayed reduced hatching when retained in the ovary at permissive temperatures, suggesting the loss of a labile egg component. This lability may also be responsible for the variable phenotypes displayed by offspring from individual females.