Introduction. The word myotonia was used for the first time by Strumpell in 1891. Diseases associated with this symptom are called myotonias. They are classified on the basis of their clinical features in: congenita myotonia, paramyotonia congenita, myotonic dystrophy and Schwartz-Jampel syndrome. Becker's myotonia is a generalized congenita nondystrophic myotonia transmitted as an autosomal recessive trait and caused by allelic mutation of the gene encoding the chloride channel CLC-1 of the skeletal muscle fiber surface membrane, localized on chromosome 7q35. At the present time, nondystrophic myotonias are referred as channelopathies. Clinical case. We describe a case of generalized myotonia in a Peruvian young male, descending of Europeans, without familial history. Conclusions. We discuss his clinical symptoms, laboratory and electrophysiologic findings, differential diagnosis, and response to the treatment with carbamazepine. We report the first case of Becker's myotonia in Peru.