K؉ currents in Drosophila muscles have been resolved into two voltage-activated currents (I A and I K ) and two Ca 2؉ -activated currents (I CF and I CS ). Mutations that affect I A (Shaker) and I CF (slowpoke) have helped greatly in the analysis of these currents and their role in membrane excitability. Lack of mutations that specifically affect channels for the delayed rectifier current (I K ) has made their genetic and functional identity difficult to elucidate. With the help of mutations in the Shab K ؉ channel gene, we show that this gene encodes the delayed rectifier K ؉ channels in Drosophila. Three mutant alleles with a temperature-sensitive paralytic phenotype were analyzed. Analysis of the ionic currents from mutant larval body wall muscles showed a specific effect on delayed rectifier K ؉ current (I K ). Two of the mutant alleles contain missense mutations, one in the aminoterminal region of the channel protein and the other in the pore region of the channel. The third allele contains two deletions in the amino-terminal region and is a null allele. These observations identity the channels that carry the delayed rectifier current and provide an in vivo physiological role for the Shab-encoded K ؉ channels in Drosophila. The availability of mutations that affect I K opens up possibilities for studying I K and its role in larval muscle excitability.