The nonA gene of Drosophila melanogaster influences vision, courtship song and viability, and encodes a protein inferred to bind single-stranded nucleic acids. This study describes the molecular and functional characterisation of the nonA gene of Drosophila virilis. The main goal of the project was to establish whether the D. virilis nonA gene rescued the viability and behavioural defects of nonA(-) mutants, and whether it carried species-specific information concerning the lovesong. The D. virilis nonA gene was cloned and sequenced. The overall structure of the D. virilis nonA gene was similar to that of D. melanogaster. Nucleotide and amino acid sequence comparisons revealed a highly diverged and repetitive N-terminus, followed by a conserved C-terminal region. Study of the promoter region highlighted islands of homology containing putative transcription factor binding sites. In addition, the D. virilis gene was found to display an unusual codon usage bias in comparison to other D. virilis or D. melanogaster genes. Finally, protein secondary structure predictions revealed differences between the D. virilis and D. melanogaster polypeptides. P element-mediated transformation was used to assess the ability of the D. virilis nonA clone to rescue viability, visual and song defects of D. melanogaster flies lacking nonA (nonA(-)) and an adjacent and partially overlapping lethal gene l(l)il9e. Complete rescue of the viability indicated that the transgene contained an entire functional D. virilis l(l)il9e gene. Visual studies assessing the transformant flies' optomotor response suggested that the D. virilis transgene completely rescued this phenotype. Lovesong analysis initially presented difficulties in distinguishing between incomplete rescue or transfer of species-species song characteristics. However, extensive statistical analyses suggested that some D. virilis song traits may be conveyed to transformants by the D. virilis nonA gene, but the large differences between the D. virilis and D. melanogaster lovesongs are probably due to the influence of many other genes.