The torpedo (top) locus of Drosophila encodes the fruitfly homolog of the vertebrate epidermal growth factor receptor gene and the neu proto-oncogene. We have isolated 13 top alleles in a screen for mutations failing to complement the female sterility of top, a recessive maternal effect allele that disrupts the establishment of the dorsoventral pattern of the egg shell and embryo. Several alleles recovered in this screen are zygotic lethal mutations; genetic analysis of these alleles has demonstrated that top is allelic to the embryonic lethal locus faint little ball. The 13 mutations recovered in our screens and 19 previously isolated top alleles have been genetically characterized through complementation tests with a series of hypomorphic and amorphic alleles. Nearly every top allele fails to complement the maternal effect sterility of top. Complementation tests show that the gene is required not only for oogenesis and embryogenesis, but also for pupal viability, for the growth of certain imaginal discs and for the patterning of specific ectodermal derivatives of the imaginal discs. Complementation analysis further demonstrates that the top lesions can be divided into general phenotypic categories: alleles affecting all gene activities in a coordinate manner, alleles preferentially affecting embryogenesis, alleles preferentially retaining oogenesis activity and alleles differentially affecting the development of specific imaginal disc derivatives. Correlations observed between the various developmental defects produced by top lesions suggest that the gene possesses several differentially, though not independently, mutable activities.