Mesoderm induction has been studied in many systems and some of the factors involved have been identified. Although the heart is mesodermal in origin, the molecular basis of heart development is essentially unknown. The Drosophila heart is a simple tubular structure similar to the early heart tube in vertebrates. The homeobox gene, tinman, has been shown to be crucial for heart formation in Drosophila. Several genes with considerable sequence similarities to tinman are expressed in cardiac primordial tissue of vertebrates and are likely to be required for heart development of higher organisms as well. In addition to transcriptional control factors, heart development might also depend on inductive signals. Here, we demonstrate that the gene wingless (wg), which is known to specify segmental polarity and neuroblast identity in Drosophila, has a novel role in mesoderm development: wg function is specifically required for heart development. A temperature-sensitive mutation of wg was used to inactivate wg function during precise developmental time periods. Elimination of wg function for a short time period after gastrulation results in the selective loss of heart precursors, without significantly affecting the formation of the body wall or visceral muscles, although some pattern defects are observed. This developmental requirement of wg for cardiac organogenesis is distinct from its function in segmentation and neurogenesis. We conclude that wg signaling is a crucial component of heart formation.