Mutations and pharmacological agents have been used to resolve and analyze several K(+) currents in Drosophila. Mutations that affect channels carrying the voltage-activated I(A) and the Ca(2+) -activated I(CF) have helped greatly in analyzing the structure, function and regulation of these channels. We now report mutations that selectively affect the delayed rectifier current, I(K). Flies mutagenized with ethylmethanesulfonate were screened for temperature-induced paralysis. Paralytic mutants identified in the screen were examined for K(+) currents in the larval body-wall muscles. The z66 mutant larvae showed a significant reduction in I(K). The mutation did not affect other K(+) currents (I(A), I(CF), or I(CS) ) or the Ca(2+) channel current in the muscles. Another mutation, z4, which showed reduced I(K), failed to complement z66. Genetic analysis localized the gene disrupted by z66 and z4 to the left arm of chromosome 3, in the 63A1-63B6 region on polytene chromosomes. The z66 and the z4 mutations, which lie in the Shab K(+) channel gene, provide an opportunity to undertake analysis of the functioning of these channels and to study the role of these channels in membrane excitability.