Identifying genetic components is an essential step toward understanding complex developmental processes. The primitive heart of the fruit fly, the dorsal vessel, which is a hemolymph-pumping organ, has provided a unique model system to identify cardiogenic genes and to further our understanding of the molecular mechanisms of cardiogenesis. Using RNA interference in developing Drosophila embryos, we performed a genomewide search for cardiogenic genes. Through analyses of the >5800 genes that cover approximately 40% of all predicted Drosophila genes, we identified a variety of genes encoding transcription factors and cell signaling proteins required for different steps during heart development. Analysis of mutant heart phenotypes and identified genes suggests that the Drosophila heart tube is segmentally patterned, like axial patterning, but assembled with regional modules. One of the identified genes, simjang, was further characterized. In the simjang mutant embryo, we found that within each segment a subset of cardial cells is missing. Interestingly, the simjang gene encodes a protein that is a component of the chromatin remodeling complex recruited by methyl-CpG-DNA binding proteins, suggesting that epigenetic information is crucial for specifying cardiac precursors. Together, these studies not only identify key regulators but also reveal mechanisms underlying heart development.