AbstractCase DescriptionA 10‐month‐old castrated male domestic longhair cat was evaluated for increasing frequency of episodic limb rigidity.Clinical FindingsThe cat presented for falling over and lying recumbent with its limbs in extension for several seconds when startled or excited. Upon examination, the cat had hypertrophied musculature, episodes of facial spasm, and a short‐strided, stiff gait.DiagnosticsElectromyography (EMG) identified spontaneous discharges that waxed and waned in amplitude and frequency, consistent with myotonic discharges. A high impact 8‐base pair (bp) deletion across the end of exon 3 and intron 3 of the chloride voltage‐gated channel 1 (CLCN1) gene was identified using whole genome sequencing.Treatment and OutcomePhenytoin treatment was initiated at 3 mg/kg po q24 h and resulted in long‐term improvement.Clinical RelevanceThis novel mutation within the CLCN1 gene is a cause of myotonia congenita in cats and we report for the first time its successful treatment.