mir-1^Δ31kb homozygotes are 100% lethal, with approximately 33% dying at embryonic stages, 33% hatching, and the remaining dying at larval stages. Homozygous mutant larvae are abnormally lethargic compared with their heterozygous siblings before death. Nearly half of all mir-1^Δ31kb mutant embryos display severe defects in muscle gene expression, indicating a late requirement for mir-1 to maintain muscle differentiation. They exhibit varying degrees of gaps in the rows of cardial cells that constitute the dorsal vessel, consistent with the requirement of mir-1 for determination and/or differentiation of cardiac cells. They exhibit defects in somatic muscle and heart formation, including frequent loss of cardioblasts and DA1 dorsal muscles.