The basic helix-loop-helix (bHLH) proteins represent an evolutionary conserved class of transcription factors that are known to play important roles in cell determination and differentiation during animal embryonic development. Following an exhaustive search of the complete Drosophila genome sequence using a PSI-BLAST strategy, we identified 19 new genes, bringing the total number of bHLH-encoding genes in the Drosophila genome to 56. These new genes belong to various subfamilies of bHLH transcription factors, such as the Daughterless, Hairy-Enhancer of split, bHLH-PAS or bHLHZip subfamilies. The embryonic expression pattern of each of these new genes has been analyzed by in situ hybridization. By looking for closely structurally related motifs, we found two genes that represent likely orthologues of vertebrate Mnt and Mlx. Together with previous reports, our data suggest that, similar to networks involved in neurogenesis and myogenesis, the network of Myc-related genes has been globally conserved throughout evolution.