Intracellular microelectrode recordings from the cervical giant fiber of normal Drosophila show a characteristic action potential waveform for this identified neuron. The action potential has a rapid initial spike followed by a prominent depolarizing afterpotential. Pharmacological experiments suggest that the giant fiber action potential depends on inward currents carried by Na+ and outward currents carried by K+. Abnormal action potentials are seen in Shaker (Sh) mutant Drosophila. This study compares the effects of six Sh alleles. In each case, abnormalities are limited to action potential repolarization. There are, however, allelic differences. Five alleles cause delayed repolarization and increased action potential durations. Going from most to least extreme, these alleles are: Sh102 greater than ShKS133 greater than ShM greater than ShE62 greater than ShrKO120. Compared to normal action potentials, durations in the extreme mutants are longer by an order of magnitude or more. One mutant allele, Sh5 appears to cause an incompletely repolarized action potential, rather than a repolarization delay.